[fuzzer] Handle more than 256 parameters of the same type

The multi-return fuzzer was able to generate more than 256 parameters of
the same type. However, the fuzzer itself could not deal with so many
parameters. With this change more than 256 parameters of the same type
can be handled and tested.

R=clemensh@chromium.org

Bug: chromium:807862
Change-Id: I6941eb0ff7e78a8feebc437624fa100adeda4e3d
Reviewed-on: https://chromium-review.googlesource.com/897673Reviewed-by: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51089}

test/fuzzer/multi-return.cc

@@ -282,7 +282,7 @@ extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size) {
       returns[i] = Constant(callee, type, 42);
       outputs[i] = 42;
     } else {
-      int n = input.NextInt8(counts[index(type)]);
+      int n = input.NextInt32(counts[index(type)]);
       int k = 0;
       while (desc->GetParameterType(k) != desc->GetReturnType(i) || --n > 0) {
         ++k;