[libFuzzer] set libFuzzer's own SEGV handler even one is already present, but...

[libFuzzer] set libFuzzer's own SEGV handler even one is already present, but call that handler from ours (unless we are unprotecting lazy counters). Call ProtectLazyCounters later, so that it runs after the initialization code in the target. 

llvm-svn: 352713