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

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

Reviewers: manojgupta, metzman

Subscribers: dberris, #sanitizers, llvm-commits

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D71031

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

Author: Dor1s

compiler-rt/include/fuzzer/FuzzedDataProvider.h

@@ -263,6 +263,12 @@ class FuzzedDataProvider {
     // 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);