[compiler-rt] FuzzedDataProvider: do not call memcpy on empty vector.

[Dor1s]

Summary

Some versions of memcpy mark pointer arguments as __nonnull, that triggers UBSan
errors even when the length passed is 0.

[Dor1s]

• remove empty line

[Dor1s]

TBR simple change.

[metzman]

LGTM.
Thanks!

[Dor1s]

• [compiler-rt] FDP: assert that num_bytes_to_consume == 0 when size == 0.

[Dor1s]

rebase

[Dor1s]

squash

[Dor1s]

another rebase

compiler-rt/include/fuzzer/FuzzedDataProvider.h

std::vector<T> ConsumeBytes(size_t size, size_t num_bytes_to_consume) {
 
     // The point of using the size-based constructor below is to increase the
     // odds of having a vector object with capacity being equal to the length.
     // That part is always implementation specific, but at least both libc++ and
     // libstdc++ allocate the requested number of bytes in that constructor,
     // which seems to be a natural choice for other implementations as well.
     // To increase the odds even more, we also call |shrink_to_fit| below.
     std::vector<T> result(size);
+    if (size == 0) {
+      if (num_bytes_to_consume != 0)
+        abort();
+      return result;
+    }
+
     std::memcpy(result.data(), data_ptr_, num_bytes_to_consume);
     Advance(num_bytes_to_consume);
 
     // Even though |shrink_to_fit| is also implementation specific, we expect it
     // to provide an additional assurance in case vector's constructor allocated
     // a buffer which is larger than the actual amount of data we put inside it.
     result.shrink_to_fit();
     return result;