Xserver/hw/nxagent/Clipboard.c: Fix free calls (free back to XFree).

 They have been flawed ever since nxagent came up, as they were Xfree
 (non-capital f) always, but should have been XFree (capital F, defined
 in Xlibint.h and part of libNX_X11).
 .
 Probably this all should be free() all over the code (bearing in mind,
 that XFree() returns int, not void. But still...

 Fixes #553.

nx-X11/programs/Xserver/hw/nxagent/Clipboard.c

@@ -754,7 +754,7 @@ void nxagentCollectPropertyEvent(int resource)
 
         if (pszReturnData != NULL)
         {
-          free(pszReturnData);
+          XFree(pszReturnData);
         }
 
         return;
@@ -796,7 +796,7 @@ void nxagentCollectPropertyEvent(int resource)
 
         if (pszReturnData != NULL)
         {
-          free(pszReturnData);
+          XFree(pszReturnData);
         }
 
         return;
@@ -974,7 +974,7 @@ void nxagentNotifySelection(XEvent *X)
           }
 
           /*
-           * free(pszReturnData);
+           * XFree(pszReturnData);
            * pszReturnData=NULL;
            */