[sanitizer] Fix handling of edge cases in mbstowcs-like interceptors.


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@186002 91177308-0d34-0410-b5e6-96231b3b80d8
[sanitizer] More checks in mbstowcs-like interceptors.


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@186004 91177308-0d34-0410-b5e6-96231b3b80d8