mesa: Return ZeroVec/dummyReg instead of NULL pointer

mesa: Return ZeroVec/dummyReg instead of NULL pointer

Assertions are not sufficient to check for null pointers as they don't
show up in release builds. So, return ZeroVec/dummyReg instead of NULL
pointer in get_{src,dst}_register_pointer(). This should calm down the
warnings from static analysis tool.

Note: This is a candidate for the 9.1 branch.
Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Brian Paul <brianp@vmware.com>