MC: loosen an overzealous assertion

Permit active macro expansions when terminating the assembler if there were
errors during the expansion.  This would only trigger on invalid input when
built with assertions.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@209309 91177308-0d34-0410-b5e6-96231b3b80d8