Fix Xcode analyze warning

[baoshan]

Below analyze hints can be found:

BFTask.m:89:12: nil passed to a callee that requires a non-null 1st parameter.

...
return [[self alloc] initWithResult:result];

This PR fix the analyze warning. Please check if the diff makes any sense.

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[nlutsenko]

Hey @baoshan, thank you for fixing it and welcome to the list of contributors.
This is good to go and should have been that way in the first place, since the public API declares the result in the class contructor as billable anyway.