fix nullptr-with-nonzero-offset in folly/io/IOBuf.cpp

Reviewed By: yfeldblum

Differential Revision: D30232471

fbshipit-source-id: 1137e306fca34f687c99c7f2155a03515926da45

folly/io/IOBuf.cpp

@@ -594,7 +594,7 @@ IOBuf::IOBuf(
       capacity_(capacity),
       flagsAndSharedInfo_(flagsAndSharedInfo) {
   assert(data >= buf);
-  assert(data + length <= buf + capacity);
+  assert(intptr_t(data) + length <= intptr_t(buf) + capacity);
 
   CHECK(!folly::asan_region_is_poisoned(buf, capacity));
 }