Fix A/52 decoding when liba52 is not present

62eefa47 introduced the same hack for
DTS already a while ago. This allows falling back to lavc gracefully.

modules/codec/a52.c

@@ -159,6 +159,9 @@ static int OpenCommon( vlc_object_t *p_this, bool b_packetizer )
 
 static int OpenDecoder( vlc_object_t *p_this )
 {
+    /* HACK: Don't use this codec if we don't have an dts audio filter */
+    if( !module_exists( "a52tofloat32" ) )
+        return VLC_EGENERIC;
     return OpenCommon( p_this, false );
 }