eglGetProcAddressImpl(): Validate procname is non-null

The EGL spec states:

> procName must be a NULL-terminated string

Add validation that procname is non-null before dereferencing it to
prevent crashing due to a bad user input. This is not required by the
EGL spec, but we are explicitly choosing to fail gracefully (return
nullptr) rather than performing undefined behavior (crashing).

Bug: b/279980674
Flag: EXEMPT bugfix
Test: angle_end2end_tests \
  --gtest_filter=EGLReadinessCheckTest.GetProcAddressNullInput/*
Change-Id: Ibcfacdacd3a7ffbb3c255ce3d975764d1f048970