[Posix] Fix DeallocFixed crashes / memory leaks

BaseHeap::Dispose() walks the page table and calls
DeallocFixed(addr, 0, kRelease) for every allocated page region.
These addresses are within file-backed view mappings (i.e. guest memory).
On Windows, VirtualFree on file-mapped pages silently returns FALSE.
On POSIX, the code hit assert_always() and crashed. Changed to return false
to match Windows behavior.

Memory leaks from munmap(addr, 0):

Callers passed length=0 to DeallocFixed(kRelease). On Windows this works because
VirtualFree(addr, 0, MEM_RELEASE) means "release the entire region" — Windows
ignores the length parameter for MEM_RELEASE and always frees the whole
allocation. On POSIX, munmap(addr, 0) fails with EINVAL, so the memory was
silently leaked every time.

src/xenia/base/memory_posix.cc

@@ -171,7 +171,7 @@ bool DeallocFixed(void* base_address, size_t length,
         case DeallocationType::kDecommit:
           return Protect(base_address, length, PageAccess::kNoAccess);
         case DeallocationType::kRelease:
-          assert_always("Error: Tried to release mapped memory!");
+          return false;
         default:
           assert_unhandled_case(deallocation_type);
       }