access/alsa.c: use NULL iso 0 with pointers.

Use NULL instead of 0 for clearing or returning a pointer.

modules/access/alsa.c

@@ -372,7 +372,7 @@ static block_t* GrabAudio( demux_t *p_demux )
     if( !p_block )
     {
         msg_Warn( p_demux, "cannot get block" );
-        return 0;
+        return NULL;
     }
 
     p_sys->p_block = p_block;
@@ -402,7 +402,7 @@ static block_t* GrabAudio( demux_t *p_demux )
             }
             default:
                 msg_Err( p_demux, "Failed to read alsa frame (%s)", snd_strerror( i_read ) );
-                return 0;
+                return NULL;
         }
     }
     else
@@ -411,10 +411,10 @@ static block_t* GrabAudio( demux_t *p_demux )
         i_read *= p_sys->i_alsa_frame_size;
     }
 
-    if( i_read <= 0 ) return 0;
+    if( i_read <= 0 ) return NULL;
 
     p_block->i_buffer = i_read;
-    p_sys->p_block = 0;
+    p_sys->p_block = NULL;
 
     /* Correct the date because of kernel buffering */
     i_correct = i_read;