[wasm] Fix -Wsign-compare warnings.

test/unittests/wasm/module-decoder-unittest.cc

Index: test/unittests/wasm/module-decoder-unittest.cc
diff --git a/test/unittests/wasm/module-decoder-unittest.cc b/test/unittests/wasm/module-decoder-unittest.cc
index f254358fc90dfbb8923d199289eca95ff0f57cdd..c30c5c7db5e9e1f9c3e46903ed7dd526d8c61120 100644
--- a/test/unittests/wasm/module-decoder-unittest.cc
+++ b/test/unittests/wasm/module-decoder-unittest.cc
@@ -1204,7 +1204,7 @@ TEST_F(WasmModuleVerifyTest, ExportTableOne_off_end) {
       FUNC_INDEX(0),  // --
   };
 
-  for (int length = 33; length < sizeof(data); length++) {
+  for (size_t length = 33; length < sizeof(data); length++) {
     ModuleResult result = DecodeModule(data, data + length);
     EXPECT_FALSE(result.ok());
     if (result.val) delete result.val;