[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/cpu/thread_state.cc

@@ -56,9 +56,10 @@ static void* AllocateContext() {
 }
 
 static void FreeContext(void* ctx) {
-  char* true_start_of_ctx = &reinterpret_cast<char*>(
-      ctx)[-static_cast<ptrdiff_t>(xe::memory::allocation_granularity())];
-  memory::DeallocFixed(true_start_of_ctx, 0,
+  size_t granularity = xe::memory::allocation_granularity();
+  char* true_start_of_ctx =
+      &reinterpret_cast<char*>(ctx)[-static_cast<ptrdiff_t>(granularity)];
+  memory::DeallocFixed(true_start_of_ctx, granularity + sizeof(ppc::PPCContext),
                        memory::DeallocationType::kRelease);
 }