egl: don't allow eglGetConfigs to set num_configs param to a negative value

egl: don't allow eglGetConfigs to set num_configs param to a negative value

When a buffer is provided to eglGetConfigs it's supposed to set the value
of the num_config parameter to the total number of configs that have been
copied into this buffer. For some reason the EGL spec doesn't consider it
to be an error to pass this function a buffer while specifying its size to
be less than 0. Given this, one would expect this combination to result in
the num_config parameter being set to 0 but this wasn't the case. This was
due to the buffer size being copied straight into num_configs without being
clamped to 0.

This was causing the following dEQP EGL test to fail:
dEQP-EGL.functional.query_config.get_configs.get_configs_bounds

Signed-off-by: Frank Binns <frank.binns@imgtec.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>