snd-card-parser: avoid virtual card info returned improperly

When getting card info with hw card(0), virtual card info may
be returned unexpectedly, causing ref count goes to bad state
for virtual card info.

Set card_def to NULL if no matching card info found in card
list.

Change-Id: I28c7d9638ac6b705b46a119aff3aaf47f9dd311a
diff --git a/snd_parser/src/snd-card-parser.c b/snd_parser/src/snd-card-parser.c
index 525028d..37824e6 100644
--- a/snd_parser/src/snd-card-parser.c
+++ b/snd_parser/src/snd-card-parser.c
@@ -491,6 +491,7 @@
         }
     }
 
+    card_def = NULL;
     /* read XML */
     file = fopen(CARD_DEF_FILE, "r");
     if (!file) {