* dri3: Check for dummyContext to see if the glx_context is valid
According to the comments in src/glx/glxcurrent.c
__glXGetCurrentContext() always returns a valid pointer. If no
context is made current, it will contain dummyContext. Thus a
test for NULL will always fail.
OBS-URL: https://build.opensuse.org/package/show/X11:XOrg/Mesa?expand=0&rev=511
