[Libva] vaDisplayIsValid w/t magic number

[Yuan, Shengquan]

There is a VA test case which passes an illegal va_dpy, e.g.
vaMaxNumConfigAttributes(0x34534). vaDisplayIsValid check whether
va_dpy==NULL, if not then call dpy->vaIsValid, and it will get a
SEGFAULT

We can avoid it with a stupid magic number, then every backend need
use the same magic number:

diff --git a/src/va.c b/src/va.c
index 6bc9c06..2cdbe02 100644
--- a/src/va.c
+++ b/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(pDisplayContex
 }

 static void va_errorMessage(const char *msg, ...)
diff --git a/src/va_backend.h b/src/va_backend.h
index 6f858f5..e4faa79 100755
--- a/src/va_backend.h
+++ b/src/va_backend.h
@@ -398,6 +398,8 @@ struct VADriverContext

 struct VADisplayContext
 {
+    int vadpy_magic;
+
     VADisplayContextP pNext;
     VADriverContextP pDriverContext;

diff --git a/src/x11/va_x11.c b/src/x11/va_x11.c
index 5908fbe..42adb9e 100644
--- a/src/x11/va_x11.c
+++ b/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;