stw/wgl: add null context check in wglBindTexImageARB()

To avoid dereferencing a null pointer in case wglMakeCurrent() wasn't
called.  Found while debugging SWKOTOR game.

Reviewed-by: Neha Bhende <bhenden@vmware.com>
Reviewed-by: Charmaine Lee <charmainel@vmware.com>

src/gallium/state_trackers/wgl/stw_ext_rendertexture.c

@@ -129,6 +129,12 @@ wglBindTexImageARB(HPBUFFERARB hPbuffer, int iBuffer)
     * we do here.
     */
 
+   if (!curctx) {
+      debug_printf("No rendering context in wglBindTexImageARB()\n");
+      SetLastError(ERROR_INVALID_OPERATION);
+      return FALSE;
+   }
+
    fb = stw_framebuffer_from_HPBUFFERARB(hPbuffer);
    if (!fb) {
       debug_printf("Invalid pbuffer handle in wglBindTexImageARB()\n");