[Coverity 1665590] Dereference null return value in input/meta.c

If the function actually returns a null value, a null pointer dereference will occur.

In vlc_meta_Merge: Return value of function which returns null is dereferenced without checking (CWE-476)

231    for( int i = 0; ppsz_all_keys && ppsz_all_keys[i]; i++ )
232    {
233        /* Always try to remove the previous value */
234        struct vlc_meta_value *dst_meta_value = vlc_dictionary_value_for_key( &dst->extra_tags, ppsz_all_keys[i] );
      14. returned_null: vlc_dictionary_value_for_key returns NULL (checked 22 out of 23 times).[show details]
      15. var_assigned: Assigning: src_meta_value = NULL return value from vlc_dictionary_value_for_key.
235        struct vlc_meta_value *src_meta_value = vlc_dictionary_value_for_key( &src->extra_tags, ppsz_all_keys[i] );
      16. Condition dst_meta_value, taking false branch.
236        if( dst_meta_value )
237        {
238            /* overwrite metadata only when priority of src is 
239               greater than or equal to the priority of dst */
      CID 1665590:(#1 of 2):Dereference null return value (NULL_RETURNS) [ "select issue" ]
240            if( src_meta_value->priority < dst_meta_value->priority )
241                continue;
242
243            vlc_dictionary_remove_value_for_key( &dst->extra_tags, ppsz_all_keys[i], vlc_meta_FreeExtraKey, NULL );
244        }
245
     
CID 1665590: (#2 of 2): Dereference null return value (NULL_RETURNS)
17. dereference: Dereferencing src_meta_value, which is known to be NULL.
246        vlc_meta_InsertExtra( dst, ppsz_all_keys[i], src_meta_value->value, src_meta_value->priority );