Fix crash when using MSVC in debug mode
Fix crash when using MSVC in debug mode

Accessing inputs_[0] when it's empty results in an assert when running
in debug. Avoid it by using data() if available.

sgraham/ninja,sgraham/ninja,bradking/ninja,Qix-/ninja,maruel/ninja,iwadon/ninja,AoD314/ninja,ninja-build/ninja,ndsol/subninja,atetubou/ninja,ninja-build/ninja,AoD314/ninja,sgraham/ninja,iwadon/ninja,ndsol/subninja,Qix-/ninja,bradking/ninja,bradking/ninja,iwadon/ninja,sgraham/ninja,atetubou/ninja,ninja-build/ninja,Qix-/ninja,iwadon/ninja,ndsol/subninja,maruel/ninja,atetubou/ninja,ninja-build/ninja,Qix-/ninja,AoD314/ninja,maruel/ninja,AoD314/ninja,maruel/ninja,atetubou/ninja,bradking/ninja,ndsol/subninja