Do not pass NULL paramenter to bsearch with empty filelist

A filelist can be non-NULL but empty (particularly with a lot of
NoExtract entries). Handle this in alpm_filelist_contains()

Identified using the undefined behaviour sanitizer.

Signed-off-by: Allan McRae <allan@archlinux.org>

lib/libalpm/filelist.c

@@ -123,7 +123,7 @@ alpm_file_t SYMEXPORT *alpm_filelist_contains(const alpm_filelist_t *filelist,
 {
 	alpm_file_t key;
 
-	if(!filelist) {
+	if(!filelist || filelist->count == 0) {
 		return NULL;
 	}