Fix read_mailmap to handle a caller uninterested in repo abbreviation

The only such a caller builtin-blame.c would pass NULL as the place
where to store the abbreviation.

Signed-off-by: Alex Riesen <raa.lkml@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>

mailmap.c

@@ -17,6 +17,9 @@ int read_mailmap(struct path_list *map, const char *filename, char **repo_abbrev
 			int abblen = sizeof(abbrev) - 1;
 			int len = strlen(buffer);
 
+			if (!repo_abbrev)
+				continue;
+
 			if (len && buffer[len - 1] == '\n')
 				buffer[--len] = 0;
 			if (!strncmp(buffer, abbrev, abblen)) {