GlobalISel: Improve crash on invalid mapping

If NumBreakDowns is 0, BreakDown is null.
This trades a null dereference with an assert somewhere
else.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@349464 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed