Initialize pointer in string conversion helpers to prevent MSAN diagnostic.

Since the initialization of the pointer happens across the libc library boundry
MSAN will not know the pointer was initialized. This fixes MSAN failures in
test/strings/string.conversions.


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@222052 91177308-0d34-0410-b5e6-96231b3b80d8