connectd: pass correct buflen to memmem

After the first iteration of the loop, we call memmem with a buflen that
points past the end of buf.

In practice we probably never read the uninitialized memory since we
guarantee the buffer ends with "\r\n", and since most/all libc
implementations probably read the haystack sequentially. But maybe
there's some libc with a crazy optimization out there. It's good to use
an accurate buflen just in case.

Discovered this while running some unit tests with MSan.

connectd/websocketd.c

@@ -109,6 +109,7 @@ static const char *get_http_hdr(const tal_t *ctx, const u8 *buf, size_t buflen,
 		    && buf[strlen(hdrname)] == ':')
 			break;
 		buf = end + 2;
+		buflen -= hdrlen + 2;
 	}
 
 	buf += strlen(hdrname) + 1;