Fixed crash caused by error handling in -invoke:…

Yes, again.

Method signatures differ between invocations, but the last argument is always where the NSError lives. Previous implementation used '5' hard-coded as the argument for the error, but this (obviously) caused a crash when using a method signature that had the error at a different position.
Now, we examine the method signature, find out the number of arguments, subtract 1 as it's zero-based, and put the error there.

UAGithubEngine/UAGithubEngine.m

@@ -334,11 +334,12 @@ - (id)sendRequest:(NSString *)path requestType:(UAGithubRequestType)requestType
 - (void)invoke:(void (^)(id obj))invocationBlock success:(UAGithubEngineSuccessBlock)successBlock failure:(UAGithubEngineFailureBlock)failureBlock
 {
     NSError __unsafe_unretained *error = nil;
-    NSError __unsafe_unretained **errorPointer = &error;
+    NSError * __unsafe_unretained *errorPointer = &error;
     id __unsafe_unretained result;
 
     NSInvocation *invocation = [NSInvocation jr_invocationWithTarget:self block:invocationBlock];
-    [invocation setArgument:&errorPointer atIndex:5];
+    // Method signatures differ between invocations, but the last argument is always where the NSError lives
+    [invocation setArgument:&errorPointer atIndex:[[invocation methodSignature] numberOfArguments] - 1];
     [invocation invoke];
     [invocation getReturnValue:&result];
 
@@ -356,11 +357,11 @@ - (void)invoke:(void (^)(id obj))invocationBlock booleanSuccess:(UAGithubEngineB
 {
 
     NSError __unsafe_unretained *error = nil;
-    NSError __unsafe_unretained **errorPointer = &error;
+    NSError * __unsafe_unretained *errorPointer = &error;
     BOOL result;
 
     NSInvocation *invocation = [NSInvocation jr_invocationWithTarget:self block:invocationBlock];
-    [invocation setArgument:&errorPointer atIndex:5];
+    [invocation setArgument:&errorPointer atIndex:[[invocation methodSignature] numberOfArguments] - 1];
     [invocation invoke];
     [invocation getReturnValue:&result];