YaGL: Prevent potential null pointer dereference in yagl_egl_release_current_context()

YaGL: Prevent potential null pointer dereference in yagl_egl_release_current_context()

This fixes a potential NULL pointer dereference issue as pointed out
by SVACE warning:

* DEREF_OF_NULL.ASSIGN: Pointer 'egl_api_ts', which is dereferenced
   at yagl_host_egl_calls.c:184, may have NULL value.
   [dereference] Dereference at hw/yagl/yagl_apis/egl/yagl_host_egl_calls.c:184
   [null] Assign null at hw/yagl/yagl_apis/egl/yagl_host_egl_calls.c:182

Change-Id: I5003403ebc881fb2ebc91d9e1be132ff4d87f2e3
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>