Avoid pointer UB in strip_header

Don't calculate header_start if lc_header_start is NULL, as we're
going to overflow the address space in that case.

Author: nikic

ext/standard/http_fopen_wrapper.c

@@ -86,11 +86,10 @@ static inline void strip_header(char *header_bag, char *lc_header_bag,
 		const char *lc_header_name)
 {
 	char *lc_header_start = strstr(lc_header_bag, lc_header_name);
-	char *header_start = header_bag + (lc_header_start - lc_header_bag);
-
 	if (lc_header_start
 	&& (lc_header_start == lc_header_bag || *(lc_header_start-1) == '\n')
 	) {
+		char *header_start = header_bag + (lc_header_start - lc_header_bag);
 		char *lc_eol = strchr(lc_header_start, '\n');
 		char *eol = header_start + (lc_eol - lc_header_start);