Avoid accessing LLVM/DWARF register mappings if undefined

Summary:
If the backend does not define LLVM/DWARF register mappings, the associated
variables are undefined since the map initializer is called by auto-generated
TableGen routines. This patch initializes the pointers and sizes to nullptr
and zero, respectively, and checks that they are valid before searching
for a mapping.

Reviewers: grosbach, dschuff

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D23458

llvm-svn: 278574