Handle glGetString returning NULL

When two contexts are shared, and one is lost/reset, the other can be
lost as well. A lost context can return NULL from glGetString. On the
first eglMakeCurrent to a context, we fetch the extension list, and
weren't checking for NULL, leading to a crash in the rare case where a
context is lost and then another context sharing with it is made
current for the first time.

Bug: 64024627
Test: dEQP-EGL.functional.robustness.reset_context.shaders.infinite_loop.shared_context_status.*
Change-Id: Ib7e1b932490ec587df159e9e461c211d3dac35d4