non-void function 'vm_loop' should return a value

[akosthekiss]

OSX Travis revealed a problem in vm_loop (in jerry-core/vm/vm.c):

../../../jerry-core/vm/vm.c:1351:13: error: non-void function 'vm_loop' should return a value [-Wreturn-type]
            return;
../../../jerry-core/vm/vm.c:1383:13: error: non-void function 'vm_loop' should return a value [-Wreturn-type]
            return;

Interestingly, gcc on Linux did not compain about it (neither on Travis, nor locally), but it is an issue nevertheless.

[akosthekiss]

Fixed by #931