SignalProducer.combineLatestWith: added workaround for #2751
SignalProducer.combineLatestWith: added workaround for #2751

Running tests with -O (and ensuring `ENABLE_TESTABILITY` is `YES`) crash.

walkingsmarts/ReactiveCocoa,sandyway/ReactiveCocoa,ikesyo/ReactiveCocoa,ohwutup/ReactiveCocoa,tzongw/ReactiveCocoa,victorlin/ReactiveCocoa,ReactiveCocoa/ReactiveSwift,RuiAAPeres/ReactiveCocoa,rpowelll/ReactiveCocoa,smilypeda/ReactiveCocoa,walkingsmarts/ReactiveCocoa,smilypeda/ReactiveCocoa,rpowelll/ReactiveCocoa,ohwutup/ReactiveCocoa,Juraldinio/ReactiveCocoa,ReactiveCocoa/ReactiveSwift,sandyway/ReactiveCocoa,Juraldinio/ReactiveCocoa,calebd/ReactiveCocoa,ohwutup/ReactiveCocoa,RuiAAPeres/ReactiveCocoa,victorlin/ReactiveCocoa,rpowelll/ReactiveCocoa,Juraldinio/ReactiveCocoa,tzongw/ReactiveCocoa,RuiAAPeres/ReactiveCocoa,andersio/ReactiveCocoa,smilypeda/ReactiveCocoa,walkingsmarts/ReactiveCocoa,victorlin/ReactiveCocoa,calebd/ReactiveCocoa,andersio/ReactiveCocoa,tzongw/ReactiveCocoa,ReactiveCocoa/ReactiveSwift,andersio/ReactiveCocoa,ikesyo/ReactiveCocoa,calebd/ReactiveCocoa,sandyway/ReactiveCocoa