mesa: asst. clean-ups in copy_label()

mesa: asst. clean-ups in copy_label()

This incorporates Vinson's change to check for a null src pointer as
detected by coverity.

Also, rename the function params to be src/dst, const-qualify src,
and use GL types to match the calling functions.  And add some more
comments.

Reviewed-by: Timothy Arceri <t_arceri@yahoo.com.au>