Check result of emitStrLen before passing it to CreateGEP

Summary:
This fixes PR43081, where the transformation of `strchr(p, 0) -> p +
strlen(p)` can cause a segfault, if `-fno-builtin-strlen` is used.  In
that case, `emitStrLen` returns nullptr, which CreateGEP is not designed
to handle.  Also add the minimized code from the PR as a test case.

Reviewers: xbolva00, spatel, jdoerfert, efriedma

Reviewed By: efriedma

Subscribers: lebedev.ri, hiraditya, cfe-commits, llvm-commits

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D70143