Fix IsUnixSocket()

This is a pure functionality fix: according to its manpage, stat()
returns 0 on success. Checking for a return value of zero fixes
incorrect results of IsUnixSocket().

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

libvncclient/rfbproto.c

@@ -348,7 +348,7 @@ static rfbBool
 IsUnixSocket(const char *name)
 {
   struct stat sb;
-  if(stat(name, &sb) && (sb.st_mode & S_IFMT) == S_IFSOCK)
+  if(stat(name, &sb) == 0 && (sb.st_mode & S_IFMT) == S_IFSOCK)
     return TRUE;
   return FALSE;
 }