[wasm] Fix fuzzer size calculation

R=ahaas@chromium.org, mythria@chromium.org
BUG=

Review-Url: https://codereview.chromium.org/2702123003
Cr-Commit-Position: refs/heads/master@{#43312}

test/fuzzer/wasm-compile.cc

@@ -59,7 +59,7 @@ class DataRange {
     } else {
       const size_t num_bytes = std::min(sizeof(T), size());
       T result;
-      memcpy(&result, data_, sizeof(num_bytes));
+      memcpy(&result, data_, num_bytes);
       data_ += num_bytes;
       size_ -= num_bytes;
       return result;