st/mesa: handle failed context creation for core profile

st/mesa: handle failed context creation for core profile

If the glx/wgl state tracker requested a core profile but the gallium
driver did not support some feature of GL 3.1 or later, we were setting
ctx->Version=0 and then failing the assertion in
_mesa_initialize_exec_table().

With this change we check for ctx->Version=0 and tear down the context
and return NULL from st_create_context().

Reviewed-by: Marek Olšák <marek.olsak@amd.com>