check_sha1_signature: check return value from read_istream

It's possible for read_istream to return an error, in which
case we just end up in an infinite loop (aside from EOF, we
do not even look at the result, but just feed it straight
into our running hash).

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

sha1_file.c

@@ -1266,6 +1266,10 @@ int check_sha1_signature(const unsigned char *sha1, void *map,
 		char buf[1024 * 16];
 		ssize_t readlen = read_istream(st, buf, sizeof(buf));
 
+		if (readlen < 0) {
+			close_istream(st);
+			return -1;
+		}
 		if (!readlen)
 			break;
 		git_SHA1_Update(&c, buf, readlen);