[CVSymbolVisitor] It's possible for an error to occur in begin()

If the begin iterator fails, we cannot dereference it's contents.
Instead, we must immediately stop processing symbols.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@271141 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed