Initialize transform manager to null in fuzzer

BUG=chromium:1167337,chromium:1167634,chromium:1167759

Change-Id: Id7eeab225bd29862e43d18f1e44c6ec8b99e26d2
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/38160
Commit-Queue: Ryan Harrison <rharrison@chromium.org>
Commit-Queue: dan sinclair <dsinclair@chromium.org>
Auto-Submit: Ryan Harrison <rharrison@chromium.org>
Reviewed-by: dan sinclair <dsinclair@chromium.org>

fuzzers/tint_common_fuzzer.cc

@@ -23,7 +23,11 @@ namespace tint {
 namespace fuzzers {
 
 CommonFuzzer::CommonFuzzer(InputFormat input, OutputFormat output)
-    : input_(input), output_(output), inspector_enabled_(false) {}
+    : input_(input),
+      output_(output),
+      transform_manager_(nullptr),
+      inspector_enabled_(false) {}
+
 CommonFuzzer::~CommonFuzzer() = default;
 
 int CommonFuzzer::Run(const uint8_t* data, size_t size) {