[scudo] Respect the return value of ReservedMemory::create() (NFC)

[cferris]

LGTM.

compiler-rt/lib/scudo/standalone/secondary.h

void *MapAllocator<Config>::allocate(const Options &Options, uptr Size,
 
   uptr RoundedSize =
       roundUp(roundUp(Size, Alignment) + getHeadersSize(), PageSize);
   if (Alignment > PageSize)
     RoundedSize += Alignment - PageSize;
 
   ReservedMemoryT ReservedMemory;
   const uptr MapSize = RoundedSize + 2 * PageSize;
-  ReservedMemory.create(/*Addr=*/0U, MapSize, nullptr, MAP_ALLOWNOMEM);
+  if (UNLIKELY(!ReservedMemory.create(/*Addr=*/0U, MapSize, nullptr,
+                                      MAP_ALLOWNOMEM))) {
+    return nullptr;
+  }
 
   // Take the entire ownership of reserved region.
   MemMapT MemMap = ReservedMemory.dispatch(ReservedMemory.getBase(),
                                            ReservedMemory.getCapacity());
   uptr MapBase = MemMap.getBase();
-  if (UNLIKELY(!MapBase))
-    return nullptr;
   uptr CommitBase = MapBase + PageSize;
   uptr MapEnd = MapBase + MapSize;
 
   // In the unlikely event of alignments larger than a page, adjust the amount
   // of memory we want to commit, and trim the extra memory.
   if (UNLIKELY(Alignment >= PageSize)) {
     // For alignments greater than or equal to a page, the user pointer (eg: the
     // pointer that is returned by the C or C++ allocation APIs) ends up on a