[MemProf] Fix bot failure about missing return value along all paths

[MemProf] Fix bot failure about missing return value along all paths

Fixes warning treated as error from
https://lab.llvm.org/buildbot/#/builders/13/builds/34440:
  'llvm::LibCallSimplifier::optimizeNew': not all control paths return a value
after a35206d78280e0ebcf48cd21bc5fff5c3b9c73fa.

Change assert in default case to a return nullptr, so we have a
reasonable fallback if invoked on a different library function, and no
longer get the above warning.