Fix GetPlatformAppByName() crash on KitKat when the app was not found

[dconeybe]

This PR fixes a bug in GetPlatformAppByName() if an exception is thrown by the Java method call. The issue is that if an exception is thrown then the return value from CallStaticObjectMethod() is NULL... except on KitKat, where it appears to return garbage instead. This garbage value was being returned and the caller was incorrectly treating it as a valid object reference. The fix is to check if an exception was thrown and explicitly returning NULL in that case, instead of relying on the return value from CallStaticObjectMethod() being NULL.

This bug surfaced in the integration test FirestoreIntegrationTest.CanPageThroughItems which was crashing on KitKat. The root cause was that it was using a custom app name and the call to GetPlatformAppByName() was returning garbage in that case.

firebase-cpp-sdk/app/src/app_android.cc

Lines 302 to 317 in 4a34edd

	static jobject GetPlatformAppByName(JNIEnv* jni_env, constchar* name) {
	jobject platform_app;
	if (app_common::IsDefaultAppName(name)) {
	platform_app = jni_env->CallStaticObjectMethod(
	app::GetClass(), app::GetMethodId(app::kGetInstance));
	} else {
	jobject name_string = jni_env->NewStringUTF(name);
	platform_app = jni_env->CallStaticObjectMethod(
	app::GetClass(), app::GetMethodId(app::kGetInstanceByName),
	name_string);
	jni_env->DeleteLocalRef(name_string);
	}
	jni_env->ExceptionCheck();
	jni_env->ExceptionClear();
	return platform_app;
	}

firebase-cpp-sdk/firestore/integration_test_internal/src/cursor_test.cc

Lines 19 to 26 in 4a34edd

	TEST_F(FirestoreIntegrationTest, CanPageThroughItems) {
	CollectionReference collection =
	Collection({{"a", {{"v", FieldValue::String("a")}}},
	{"b", {{"v", FieldValue::String("b")}}},
	{"c", {{"v", FieldValue::String("c")}}},
	{"d", {{"v", FieldValue::String("d")}}},
	{"e", {{"v", FieldValue::String("e")}}},
	{"f", {{"v", FieldValue::String("f")}}}});

[github-actions]

❌  Integration test FAILED

Requested by @dconeybe on commit 57d29a5
Last updated: Wed May 19 12:33:28 PDT 2021
View integration test results

Platform (Build Config)	Build failures	Test failures (Test Devices)
Android (built on Linux)		firestore (Nexus10+19) installations (Pixel2+28, flame+29) messaging (Nexus10+19, Pixel2+28)
Android (built on MacOS)		firestore (Nexus10+19) messaging (Nexus10+19, Pixel2+28)
Android (built on Windows)		firestore (Nexus10+19) messaging (Nexus10+19, Pixel2+28)

[dconeybe]

Note: I'm going to ignore the test failures because they are all flakes that could not possibly be triggered by this PR.