gpg-interface: fix const-correctness of "eol" pointer

We accidentally shed the "const" of our buffer by passing it
through memchr. Let's fix that, and while we're at it, move
our variable declaration inside the loop, which is the only
place that uses it.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Ben Toews <mastahyeti@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

gpg-interface.c

@@ -103,11 +103,10 @@ void print_signature_buffer(const struct signature_check *sigc, unsigned flags)
 
 size_t parse_signature(const char *buf, size_t size)
 {
-	char *eol;
 	size_t len = 0;
 	while (len < size && !starts_with(buf + len, PGP_SIGNATURE) &&
 			!starts_with(buf + len, PGP_MESSAGE)) {
-		eol = memchr(buf + len, '\n', size - len);
+		const char *eol = memchr(buf + len, '\n', size - len);
 		len += eol ? eol - (buf + len) + 1 : size - len;
 	}
 	return len;