kernel: flush input line after early syntax error

Fixes a mild regression introduce by PR #1015, which in turn attempted
to fix parts of issue #995. With GAP 4.8 and also with this fix:

    gap> 1.2\3;
    Syntax error: Badly formed number
    1.2\3;
       ^

In GAP 4.9:

    gap> 1.2\3;
    Syntax error: Badly formed number
    1.2\3;
       ^
    3

src/read.c

@@ -2713,8 +2713,9 @@ ExecStatus ReadEvalCommand(Obj context, Obj *evalResult, UInt *dualSemicolon)
     /* get the first symbol from the input                                 */
     Match( STATE(Symbol), "", 0UL );
 
-    /* using TRY_READ sets the jump buffer and mucks up the interpreter    */
+    // if scanning the first symbol produced a syntax error, abort
     if (STATE(NrError)) {
+        FlushRestOfInputLine();
         return STATUS_ERROR;
     }
 

tst/testinstall/kernel/read.tst

@@ -64,5 +64,71 @@ Syntax error: Identifier expected in stream:1
 rec("a":=1);
       ^
 
+#
+# ReadEvalCommand
+#
+# See also https://github.com/gap-system/gap/issues/995
+#
+
+# some inputs which immediately trigger an error in ReadEvalCommand,
+# even before an interpreter has been started
+gap> '
+Syntax error: Character literal must not include <newline> in stream:1
+'
+^
+gap> 'x
+Syntax error: Missing single quote in character constant in stream:1
+'x
+ ^
+gap> "
+Syntax error: String must not include <newline> in stream:1
+"
+^
+gap> "x
+Syntax error: String must not include <newline> in stream:1
+"x
+ ^
+
+# similar inputs to the above, but here the error is triggered a bit
+# later, after the interpreter has already started
+gap> s := '
+Syntax error: Character literal must not include <newline> in stream:1
+s := '
+     ^
+Syntax error: ; expected in stream:2
+^
+gap> s := 'x
+Syntax error: Missing single quote in character constant in stream:1
+s := 'x
+      ^
+Syntax error: ; expected in stream:2
+^
+gap> s := "
+Syntax error: String must not include <newline> in stream:1
+s := "
+     ^
+Syntax error: ; expected in stream:2
+^
+gap> s := "x
+Syntax error: String must not include <newline> in stream:1
+s := "x
+      ^
+Syntax error: ; expected in stream:2
+^
+
+# errors in the middle of parsing a float literal
+gap> 12.34\56;
+Syntax error: Badly formed number in stream:1
+12.34\56;
+     ^
+gap> 12.34\a56;
+Syntax error: Badly formed number in stream:1
+12.34\a56;
+     ^
+gap> 12.34\56a;
+Syntax error: Badly formed number in stream:1
+12.34\56a;
+     ^
+
 #
 gap> STOP_TEST("kernel/read.tst", 1);