st/wgl: check for hPbuffer=0 in wgl pbuffer functions

Per the GL_WGL_pbuffer spec, generate ERROR_INVALID_HANDLE and
return FALSE (and don't segfault).

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>

src/gallium/state_trackers/wgl/stw_ext_pbuffer.c

@@ -239,6 +239,11 @@ wglGetPbufferDCARB(HPBUFFERARB hPbuffer)
    struct stw_framebuffer *fb;
    HDC hDC;
 
+   if (!hPbuffer) {
+      SetLastError(ERROR_INVALID_HANDLE);
+      return NULL;
+   }
+
    fb = (struct stw_framebuffer *)hPbuffer;
 
    hDC = GetDC(fb->hWnd);
@@ -255,6 +260,11 @@ wglReleasePbufferDCARB(HPBUFFERARB hPbuffer,
 {
    struct stw_framebuffer *fb;
 
+   if (!hPbuffer) {
+      SetLastError(ERROR_INVALID_HANDLE);
+      return 0;
+   }
+
    fb = (struct stw_framebuffer *)hPbuffer;
 
    return ReleaseDC(fb->hWnd, hDC);
@@ -266,6 +276,11 @@ wglDestroyPbufferARB(HPBUFFERARB hPbuffer)
 {
    struct stw_framebuffer *fb;
 
+   if (!hPbuffer) {
+      SetLastError(ERROR_INVALID_HANDLE);
+      return FALSE;
+   }
+
    fb = (struct stw_framebuffer *)hPbuffer;
 
    /* This will destroy all our data */
@@ -280,6 +295,11 @@ wglQueryPbufferARB(HPBUFFERARB hPbuffer,
 {
    struct stw_framebuffer *fb;
 
+   if (!hPbuffer) {
+      SetLastError(ERROR_INVALID_HANDLE);
+      return FALSE;
+   }
+
    fb = (struct stw_framebuffer *)hPbuffer;
 
    switch (iAttribute) {