glcontext_egl: fix glcontext fail when egl support EGL_KHR_no_config_context
eglchooseconfig() will return NULL config when no valid config found, eg when try GLES 3.2 on GLES 2.0 platform, but this will not make create context fail because it support EGL_KHR_no_config_context, and will fail later.
Need also check numconfig if support EGL_KHR_no_config_context
Edited by Haihua Hu