regcomp: use long jumps if there is any possibility of overflow

(CVE-2020-10878) Be conservative for backporting, we'll aim to do
something more aggressive for bleadperl.

(cherry picked from commit 9d7759db46f3b31b1d3f79c44266b6ba42a47fc6)

regcomp.c

@@ -7762,6 +7762,13 @@ Perl_re_op_compile(pTHX_ SV ** const patternp, int pat_count,
 
         /* We have that number in RExC_npar */
         RExC_total_parens = RExC_npar;
+
+        /* XXX For backporting, use long jumps if there is any possibility of
+         * overflow */
+        if (RExC_size > U16_MAX && ! RExC_use_BRANCHJ) {
+            RExC_use_BRANCHJ = TRUE;
+            flags |= RESTART_PARSE;
+        }
     }
     else if (! MUST_RESTART(flags)) {
 	ReREFCNT_dec(Rx);