These tests were using malloc()'s return value without checking for null,
which MSVC's /analyze rightly warns about. Asserting that the pointer is
non-null both expresses the test's intention and silences the warning.
Fixes D27785.
git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@290921 91177308-0d34-0410-b5e6-96231b3b80d8