egl: Add NULL check for num_config parameter

EGL 1.5 specification says in 3.4.1:
"An EGL_BAD_PARAMETER error is generated if num config is NULL."

We have to check for the condition and return the required error to
application for eglGetConfigs and eglGetConfigs functions.

Test: refactoring CL.
Change-Id: Ib42709f0420161ce661536394d4c0779bc62be57
Signed-off-by: Pauli Nieminen <pauli.nieminen@linux.intel.com>
Signed-off-by: Liu Zhiquan <zhiquan.liu@intel.com>