Added a "magic" number to VADisplayContext in case an illeal VA dpy passed in

Signed-off-by: Austin Yuan <shengquan.yuan@intel.com>

src/va.c

@@ -58,7 +58,7 @@ static int va_debug_trace = 0;
 static int vaDisplayIsValid(VADisplay dpy)
 {
   VADisplayContextP pDisplayContext = (VADisplayContextP)dpy;
-  return pDisplayContext && pDisplayContext->vaIsValid(pDisplayContext);
+  return pDisplayContext && (pDisplayContext->vadpy_magic == 0x789abcde) && pDisplayContext->vaIsValid(pDisplayContext);
 }
 
 static void va_errorMessage(const char *msg, ...)

src/va_backend.h

@@ -398,6 +398,8 @@ struct VADriverContext
 
 struct VADisplayContext
 {
+    int vadpy_magic;
+    
     VADisplayContextP pNext;
     VADriverContextP pDriverContext;
 

src/x11/va_x11.c

@@ -189,6 +189,8 @@ VADisplay vaGetDisplay (
       dri_state       = calloc(1, sizeof(*dri_state));
       if (pDisplayContext && pDriverContext && dri_state)
       {
+	  pDisplayContext->vadpy_magic = 0x789abcde;          
+
 	  pDriverContext->x11_dpy          = native_dpy;
 	  pDisplayContext->pNext           = pDisplayContexts;
 	  pDisplayContext->pDriverContext  = pDriverContext;