gallium/dri: test 'vendor_str' before dereferencing it
Test 'vendor_str' before dereferencing it 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