fuzzers: Don't call data() on empty std::vector

UBSAN takes objection to this.

Fixed: chromium:1230344
Fixed: chromium:1230346
Fixed: chromium:1230372
Fixed: chromium:1230439
Fixed: chromium:1230457
Change-Id: I351bca06911f2e87f929f08d2aa78a1d8d43d296
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/58399
Auto-Submit: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>

fuzzers/tint_common_fuzzer.h

@@ -50,9 +50,11 @@ class Reader {
       return {};
     }
     std::vector<T> out(count);
+    if (!out.empty()) {
       memcpy(out.data(), data_, count * sizeof(T));
       data_ += count * sizeof(T);
       size_ -= count * sizeof(T);
+    }
     return out;
   }