mesa: add buffer != 0 checks to glNamedBufferEXT functions

mesa: add buffer != 0 checks to glNamedBufferEXT functions

The EXT_direct_state_access spec says:

    INVALID_OPERATION is generated by GetNamedBufferParameterivEXT,
    GetNamedBufferPointervEXT, GetNamedBufferSubDataEXT,
    MapNamedBufferEXT, NamedBufferDataEXT, NamedBufferSubDataEXT, and
    UnmapNamedBufferEXT if the buffer parameter is zero.

This commits adds buffer != 0 validation to the implemented functions.

glNamedBufferStorageEXT isn't included in this list and the EXT_buffer_storage
doesn't says that buffer = 0 is an error either so I didn't add the same
validation for this function.

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