Add binary format parse check for imported function types (#4423)

Without this we hit an assertion later, which is less clear.

See #4413

src/wasm/wasm-binary.cpp

@@ -2057,7 +2057,13 @@ void WasmBinaryBuilder::readImports() {
         Name name(std::string("fimport$") + std::to_string(functionCounter++));
         auto index = getU32LEB();
         functionTypes.push_back(getTypeByIndex(index));
-        auto curr = builder.makeFunction(name, getTypeByIndex(index), {});
+        auto type = getTypeByIndex(index);
+        if (!type.isSignature()) {
+          throwError(std::string("Imported function ") + module.str + '.' +
+                     base.str +
+                     "'s type must be a signature. Given: " + type.toString());
+        }
+        auto curr = builder.makeFunction(name, type, {});
         curr->module = module;
         curr->base = base;
         functionImports.push_back(curr.get());