utf8_to_uv_msgs: Avoid extra deref pointer writes

This function is passed an address of where to write some values.
Instead of updating the derefernced pointer each time, do it once after
all the information is accumulated.

This allows for the values to be updated without updating the pointed to
variable.

Author: khwilliamson

utf8.c

@@ -1991,6 +1991,7 @@ Perl_utf8_to_uv_msgs_helper_(const U8 * const s0,
         bool disallowed = FALSE;
         const U32 orig_problems = possible_problems;
         U32 error_flags_return = 0;
+        AV * msgs_return = NULL;
 
         /* The following macro returns 0 if no message needs to be generated
          * for this problem even if everything else says to.  Otherwise returns
@@ -2471,17 +2472,17 @@ Perl_utf8_to_uv_msgs_helper_(const U8 * const s0,
 
             } /* End of switch() on the possible problems */
 
-            /* Display the message (if any) for the problem being handled in
-             * this iteration of the loop */
+            /* Display or save the message (if any) for the problem being
+             * handled in this iteration of the loop */
             if (message) {
                 if (msgs) {
-                    if (*msgs == NULL) {
-                        *msgs = newAV();
+                    if (msgs_return == NULL) {
+                        msgs_return = newAV();
                     }
 
-                    av_push(*msgs, newRV_noinc((SV*) new_msg_hv(message,
-                                                              pack_warn,
-                                                              this_flag_bit)));
+                    av_push(msgs_return,
+                            newRV_noinc((SV*) new_msg_hv(message, pack_warn,
+                                                         this_flag_bit)));
                 }
                 else if (PL_op)
                     Perl_warner(aTHX_ pack_warn, "%s in %s", message,
@@ -2498,6 +2499,10 @@ Perl_utf8_to_uv_msgs_helper_(const U8 * const s0,
             *advance_p = curlen;
         }
 
+        if (msgs_return) {
+            *msgs = msgs_return;
+        }
+
         if (errors) {
             *errors = error_flags_return;
         }