mg.c: Fix to compile under g++

Commit 00a5df846e035280750985222a693ac58022ee36 fixed some c undefined
behavior, but it will not compile with g++, because a goto crosses
initialization.

https://stackoverflow.com/questions/14274225/statement-goto-can-not-cross-variable-definition

and its links sort of explains why

This commit works around this limitation

1 files changed, 4 insertions, 2 deletions

diff --git a/mg.c b/mg.c
index df7ad6e0ed..b3219d7623 100644
--- a/mg.c
+++ b/mg.c
@@ -867,6 +867,10 @@ Perl_magic_get(pTHX_ SV *sv, MAGIC *mg)
 
     PERL_ARGS_ASSERT_MAGIC_GET;
 
+    const char * const remaining = (mg->mg_ptr)
+                                   ? mg->mg_ptr + 1
+                                   : NULL;
+
     if (!mg->mg_ptr) {
         paren = mg->mg_len;
         if (PL_curpm && (rx = PM_GETRE(PL_curpm))) {
@@ -878,8 +882,6 @@ Perl_magic_get(pTHX_ SV *sv, MAGIC *mg)
         return 0;
     }
 
-    const char * const remaining = mg->mg_ptr + 1;
-
     nextchar = *remaining;
     switch (*mg->mg_ptr) {
     case '\001':		/* ^A */