Make sure p is not pointing to the start of the buffer

ext/standard/string.c

@@ -4263,7 +4263,7 @@ PHPAPI size_t php_strip_tags_ex(char *rbuf, int len, int *stateptr, char *allow,
 					tp = ((tp-tbuf) >= PHP_TAG_BUF_SIZE ? tbuf: tp);
 					*(tp++) = c;
 				}
-				if (*(p-1) != '\\') {
+				if (p != buf && *(p-1) != '\\') {
 					in_q = !in_q;
 				}
 				break;