Suppress Coverity warnings about Asserts in get_name_for_var_field.

Coverity thinks dpns->plan could be null at these points.  That
shouldn't really be possible, but it's easy enough to modify the
Asserts so they'd not core-dump if it were true.

These are new in b919a97a6.  Back-patch to v13; the v12 version
of the patch didn't have these Asserts.

(cherry picked from commit 16e67bc5f98f2f5691fb5c01d5a8310de1c62b81)

src/backend/utils/adt/ruleutils.c

@@ -7904,11 +7904,11 @@ get_name_for_var_field(Var *var, int fieldno,
 					{
 						char	   *dummy_name = palloc(32);
 
-						Assert(IsA(dpns->plan, Result));
+						Assert(dpns->plan && IsA(dpns->plan, Result));
 						snprintf(dummy_name, 32, "f%d", fieldno);
 						return dummy_name;
 					}
-					Assert(IsA(dpns->plan, SubqueryScan));
+					Assert(dpns->plan && IsA(dpns->plan, SubqueryScan));
 
 					tle = get_tle_by_resno(dpns->inner_tlist, attnum);
 					if (!tle)
@@ -8035,12 +8035,12 @@ get_name_for_var_field(Var *var, int fieldno,
 					{
 						char	   *dummy_name = palloc(32);
 
-						Assert(IsA(dpns->plan, Result));
+						Assert(dpns->plan && IsA(dpns->plan, Result));
 						snprintf(dummy_name, 32, "f%d", fieldno);
 						return dummy_name;
 					}
-					Assert(IsA(dpns->plan, CteScan) ||
-						   IsA(dpns->plan, WorkTableScan));
+					Assert(dpns->plan && (IsA(dpns->plan, CteScan) ||
+										  IsA(dpns->plan, WorkTableScan)));
 
 					tle = get_tle_by_resno(dpns->inner_tlist, attnum);
 					if (!tle)