[wasm] compile fuzzer: initialize temporary before filling.

BUG= https://bugs.chromium.org/p/chromium/issues/detail?id=697191

Change-Id: I01ddd6824b1a79d86944ac766f5c2070e9b0c244
Reviewed-on: https://chromium-review.googlesource.com/448317Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Eric Holk <eholk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43522}

test/fuzzer/wasm-compile.cc

@@ -57,8 +57,13 @@ class DataRange {
     if (size() == 0) {
       return T();
     } else {
+      // We want to support the case where we have less than sizeof(T) bytes
+      // remaining in the slice. For example, if we emit an i32 constant, it's
+      // okay if we don't have a full four bytes available, we'll just use what
+      // we have. We aren't concerned about endianness because we are generating
+      // arbitrary expressions.
       const size_t num_bytes = std::min(sizeof(T), size());
-      T result;
+      T result = T();
       memcpy(&result, data_, num_bytes);
       data_ += num_bytes;
       size_ -= num_bytes;