gallium/dri: check option "force_gl_vendor" first
Check option "force_gl_vendor" first before querying the option and dereferencing vendor_str to improve the robustness. It fixes an pointer dereferencing issue seen with downstream patch [1] on AOSP tree.
[1] https://android.googlesource.com/platform/external/mesa3d/+/48c0faf42671
Signed-off-by: Shawn Guo shawn.guo@linaro.org