for-each-ref: cope with tags with incomplete lines

If you have a tag with a single, incomplete line as its payload, asking
git-for-each-ref for its %(body) element accessed a NULL pointer.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-for-each-ref.c

@@ -459,8 +459,10 @@ static void find_subpos(const char *buf, unsigned long sz, const char **sub, con
 		return;
 	*sub = buf; /* first non-empty line */
 	buf = strchr(buf, '\n');
-	if (!buf)
+	if (!buf) {
+		*body = "";
 		return; /* no body */
+	}
 	while (*buf == '\n')
 		buf++; /* skip blank between subject and body */
 	*body = buf;