[libFuzzer] Fix bug in detecting timeouts when input string is empty.

I added a new flag RunningCB to know if the Fuzzer's main thread is
running the CB function, instead of using (!CurrentUnitSize).
(!CurrentUnitSize) doesn't work properly. For example, in FuzzerLoop.cpp,
inside ShuffleAndMinimize() function, we execute the callback with an
empty string (size=0). Previous implementation failed to detect timeouts
in that execution.
Also, I add a regression test for that case.

Differential Revision: https://reviews.llvm.org/D27433

llvm-svn: 289561