Small correction in reading of commit headers

Check if a line of the header has enough characters to possibly
contain the requested prefix.
Signed-off-by: NAlex Riesen <raa.lkml@gmail.com>
Signed-off-by: NJunio C Hamano <junkio@cox.net>

commit.c

@@ -640,7 +640,9 @@ static char *get_header(const struct commit *commit, const char *key)
 			next = NULL;
 		} else
 			next = eol + 1;
-		if (!strncmp(line, key, key_len) && line[key_len] == ' ') {
+		if (eol - line > key_len &&
+		    !strncmp(line, key, key_len) &&
+		    line[key_len] == ' ') {
 			int len = eol - line - key_len;
 			char *ret = xmalloc(len);
 			memcpy(ret, line + key_len + 1, len - 1);