Avparser: Fix typo and remove trailing spaces

(cherry picked from commit a56bd7ec491831228cb469af0e30c5f6d4735413)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/packetizer/avparser.c

@@ -178,7 +178,7 @@ static block_t *Packetize ( decoder_t *p_dec, block_t **pp_block )
 out:
     p_sys->i_offset = 0;
     block_Release( *pp_block );
-    *pp_block == NULL;
+    *pp_block = NULL;
     return NULL;
 }