[instance] check the instance to be valid in platform callbacks (#2005)

This commit adds a new API `otInstanceIsInitialized()` to check if a
given instance is valid, i.e, it has been initialized and not
finalized. We then use this function to verify that passed-in instance
argument in platform callbacks is valid.

src/core/api/instance_api.cpp

@@ -155,6 +155,11 @@ exit:
     return instance;
 }
 
+bool otInstanceIsInitialized(otInstance *aInstance)
+{
+    return (aInstance != NULL);
+}
+
 #else // #if OPENTHREAD_ENABLE_MULTIPLE_INSTANCES
 
 otInstance *otInstanceInitSingle(void)
@@ -181,6 +186,11 @@ exit:
     return sInstance;
 }
 
+bool otInstanceIsInitialized(otInstance *aInstance)
+{
+    return (aInstance != NULL) && (aInstance == sInstance);
+}
+
 #endif // #if OPENTHREAD_ENABLE_MULTIPLE_INSTANCES
 
 void otInstanceFinalize(otInstance *aInstance)