Move new assert to correct location

Move new assert to correct location

This assert was meant to execute at the end of parseMetadata, but
we return early and never reach the end of the function. Caught
by a compile-time warning since the function doesn't return a value
from that location.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@253762 91177308-0d34-0410-b5e6-96231b3b80d8