dix: rpcbuf: flag for auto-clear in error case

when the err_clear flag is set, the buffer memory will automatically
be free()ed when allocation failed. This allows simplificatoin of
caller's error pathes: the caller doesn't need to to call x_rpcbuf_clear()
anymore and eg. can directly return out.

Signed-off-by: Enrico Weigelt, metux IT consult <info@metux.net>

dix/rpcbuf_priv.h

@@ -32,6 +32,7 @@ typedef struct x_rpcbuf {
     char *buffer;   /* pointer to whole buffer */
     Bool swapped;   /* TRUE when typed write operation shall byte-swap */
     Bool error;     /* TRUE when the last allocation failed */
+    Bool err_clear; /* set to TRUE if should automatically clear on error */
 } x_rpcbuf_t;
 
 #define XLIBRE_RPCBUF_CHUNK_SIZE 4096