Revert "stream_filter/record.c: Read() return 0 on malloc failure."

stream_Read() is designed to allow a NULL pointer as second argument.
Therefor this commit was wrong. I added a note to avoid making the same
mistake.

This reverts commit a7ef53d4.

modules/stream_filter/record.c

@@ -121,10 +121,7 @@ static int Read( stream_t *s, void *p_read, unsigned int i_read )
     if( p_sys->f && !p_record )
         p_record = malloc( i_read );
 
-    if (!p_record)
-        return 0; /* VLC_ENOMEM */
-
-    /* */
+    /* p_record is allowed to be NULL */
     const int i_record = stream_Read( s->p_source, p_record, i_read );
 
     /* Dump read data */