[libomptarget] Initialize reference parameter IsNew within Device::getOrAllocTgtPtr

The two locals IsNew and Pointer_IsNew were uninitialized at declaration, and then passed by
reference to Device.getOrAllocTgtPtr which in turn did not assign on all
paths within the function. This resulted in occasional runtime failures in one application.
Device::getOrAllocTgtPtr will now initialize IsNew to false on entry to function.

Differential Revision: https://reviews.llvm.org/D78744