[libFuzzer] Fallback to default Mutate when MutateWithMask fails.

[Dor1s]

Summary

In case the current corpus input doesn't have bytes going into the
focus function, MutateWithMask is useless and may fail gently, allowing the
default mutation routine happen, rather than crashing on an assertion.

For more context and the initial fix suggestion, see:
https://github.com/google/oss-fuzz/issues/1632#issuecomment-481862879

lib/fuzzer/FuzzerLoop.cpp

for (int i = 0; i < Options.MutateDepth; i++) {
     if (TotalNumberOfRuns >= Options.MaxNumberOfRuns)
       break;
     MaybeExitGracefully();
     size_t NewSize = 0;
     if (II.HasFocusFunction && !II.DataFlowTraceForFocusFunction.empty() &&
         Size <= CurrentMaxMutationLen)
       NewSize = MD.MutateWithMask(CurrentUnitData, Size, Size,
                                   II.DataFlowTraceForFocusFunction);
-    else
+    
+    // If MutateWithMask either failed or wasn't called, call default Mutate.
+    if (!NewSize)
       NewSize = MD.Mutate(CurrentUnitData, Size, CurrentMaxMutationLen);
     assert(NewSize > 0 && "Mutator returned empty unit");
     assert(NewSize <= CurrentMaxMutationLen && "Mutator return oversized unit");
     Size = NewSize;
     II.NumExecutedMutations++;
 
     bool FoundUniqFeatures = false;
     bool NewCov = RunOne(CurrentUnitData, Size, /*MayDeleteFile=*/true, &II,

lib/fuzzer/FuzzerMutate.cpp

size_t MutationDispatcher::MutateWithMask(uint8_t *Data, size_t Size,
   auto &T = MutateWithMaskTemp;
   if (T.size() < Size)
     T.resize(Size);
   size_t OneBits = 0;
   for (size_t I = 0; I < Size; I++)
     if (Mask[I])
       T[OneBits++] = Data[I];
 
+  if (!OneBits) return 0;
   assert(!T.empty());
   size_t NewSize = Mutate(T.data(), OneBits, OneBits);
   assert(NewSize <= OneBits);
   (void)NewSize;
   // Even if NewSize < OneBits we still use all OneBits bytes.
   for (size_t I = 0, J = 0; I < Size; I++)
     if (Mask[I])
       Data[I] = T[J++];