regcomp.c: Don't read off the end of buffer

Until this commit, it was possible that \p{nv=3/} would cause the right
brace to be considered part of the property name.

Spotted by Hugo van der Sanden

regcomp.c

@@ -23092,7 +23092,9 @@ Perl_parse_uniprop_string(pTHX_
             }
 
             /* Store the first real character in the denominator */
-            lookup_name[j++] = name[i];
+            if (i < name_len) {
+                lookup_name[j++] = name[i];
+            }
         }
     }