pcre2_jit_compile: avoid potential wraparound if framesize <= 0

Change the minimum framesize value to match what the code can
support, while at it, refactor the conditionals to make them
safer to use.

There were unexpected hits for the logic that handles OP_ASSERT_NOT
and OP_ASSERTBACK_NOT that are still unresolved.

src/pcre2_jit_compile.c

@@ -10898,7 +10898,8 @@ if (opcode == OP_ASSERT || opcode == OP_ASSERTBACK)
     {
     JUMPTO(SLJIT_JUMP, backtrack->matchingpath);
     JUMPHERE(brajump);
-    if (framesize >= 0)
+    SLJIT_ASSERT(framesize > 0);
+    if (framesize > 1)
       {
       OP1(SLJIT_MOV, STACK_TOP, 0, SLJIT_MEM1(SLJIT_SP), private_data_ptr);
       add_jump(compiler, &common->revertframes, JUMP(SLJIT_FAST_CALL));
@@ -13504,7 +13505,7 @@ PCRE2_SPTR ccbegin;
 PCRE2_SPTR ccprev;
 PCRE2_UCHAR bra = OP_BRA;
 PCRE2_UCHAR ket;
-assert_backtrack *assert;
+const assert_backtrack *assert;
 BOOL has_alternatives;
 BOOL needs_control_head = FALSE;
 BOOL has_vreverse;
@@ -13703,14 +13704,17 @@ if (SLJIT_UNLIKELY(opcode == OP_COND) || SLJIT_UNLIKELY(opcode == OP_SCOND))
   if (ccbegin[1 + LINK_SIZE] >= OP_ASSERT && ccbegin[1 + LINK_SIZE] <= OP_ASSERTBACK_NOT)
     {
     SLJIT_ASSERT(has_alternatives);
-    assert = CURRENT_AS(bracket_backtrack)->u.assert;
-    if (assert->framesize >= 0 && (ccbegin[1 + LINK_SIZE] == OP_ASSERT || ccbegin[1 + LINK_SIZE] == OP_ASSERTBACK))
+    if (ccbegin[1 + LINK_SIZE] == OP_ASSERT || ccbegin[1 + LINK_SIZE] == OP_ASSERTBACK)
       {
-      OP1(SLJIT_MOV, STACK_TOP, 0, SLJIT_MEM1(SLJIT_SP), assert->private_data_ptr);
-      add_jump(compiler, &common->revertframes, JUMP(SLJIT_FAST_CALL));
-      OP1(SLJIT_MOV, TMP1, 0, SLJIT_MEM1(STACK_TOP), STACK(-2));
-      OP2(SLJIT_ADD, STACK_TOP, 0, STACK_TOP, 0, SLJIT_IMM, (assert->framesize - 1) * sizeof(sljit_sw));
-      OP1(SLJIT_MOV, SLJIT_MEM1(SLJIT_SP), assert->private_data_ptr, TMP1, 0);
+      assert = CURRENT_AS(bracket_backtrack)->u.assert;
+      if (assert->framesize > 1)
+        {
+        OP1(SLJIT_MOV, STACK_TOP, 0, SLJIT_MEM1(SLJIT_SP), assert->private_data_ptr);
+        add_jump(compiler, &common->revertframes, JUMP(SLJIT_FAST_CALL));
+        OP1(SLJIT_MOV, TMP1, 0, SLJIT_MEM1(STACK_TOP), STACK(-2));
+        OP2(SLJIT_ADD, STACK_TOP, 0, STACK_TOP, 0, SLJIT_IMM, (assert->framesize - 1) * sizeof(sljit_sw));
+        OP1(SLJIT_MOV, SLJIT_MEM1(SLJIT_SP), assert->private_data_ptr, TMP1, 0);
+        }
       }
     cond = JUMP(SLJIT_JUMP);
     set_jumps(CURRENT_AS(bracket_backtrack)->u.assert->condfailed, LABEL());
@@ -13887,14 +13891,17 @@ if (has_alternatives)
   if (cond != NULL)
     {
     SLJIT_ASSERT(opcode == OP_COND || opcode == OP_SCOND);
-    assert = CURRENT_AS(bracket_backtrack)->u.assert;
-    if ((ccbegin[1 + LINK_SIZE] == OP_ASSERT_NOT || ccbegin[1 + LINK_SIZE] == OP_ASSERTBACK_NOT) && assert->framesize >= 0)
+    if (ccbegin[1 + LINK_SIZE] == OP_ASSERT_NOT || ccbegin[1 + LINK_SIZE] == OP_ASSERTBACK_NOT)
       {
-      OP1(SLJIT_MOV, STACK_TOP, 0, SLJIT_MEM1(SLJIT_SP), assert->private_data_ptr);
-      add_jump(compiler, &common->revertframes, JUMP(SLJIT_FAST_CALL));
-      OP1(SLJIT_MOV, TMP1, 0, SLJIT_MEM1(STACK_TOP), STACK(-2));
-      OP2(SLJIT_ADD, STACK_TOP, 0, STACK_TOP, 0, SLJIT_IMM, (assert->framesize - 1) * sizeof(sljit_sw));
-      OP1(SLJIT_MOV, SLJIT_MEM1(SLJIT_SP), assert->private_data_ptr, TMP1, 0);
+      assert = CURRENT_AS(bracket_backtrack)->u.assert;
+      if (assert->framesize > 1)
+        {
+        OP1(SLJIT_MOV, STACK_TOP, 0, SLJIT_MEM1(SLJIT_SP), assert->private_data_ptr);
+        add_jump(compiler, &common->revertframes, JUMP(SLJIT_FAST_CALL));
+        OP1(SLJIT_MOV, TMP1, 0, SLJIT_MEM1(STACK_TOP), STACK(-2));
+        OP2(SLJIT_ADD, STACK_TOP, 0, STACK_TOP, 0, SLJIT_IMM, (assert->framesize - 1) * sizeof(sljit_sw));
+        OP1(SLJIT_MOV, SLJIT_MEM1(SLJIT_SP), assert->private_data_ptr, TMP1, 0);
+        }
       }
     JUMPHERE(cond);
     }