Fix coverity issue in block::initEmptyBlock (#1491)

getObjectSize should not return values bigger then 2^16-1.
We are assigning it's return value, which is 32 bit to 16 bit one,
so it is good to assert it anyway.

src/tbbmalloc/frontend.cpp

@@ -817,6 +817,7 @@ unsigned int getSmallObjectIndex(unsigned int size)
 /*
  * Depending on indexRequest, for a given size return either the index into the bin
  * for objects of this size, or the actual size of objects in this bin.
+ * TODO: Change return type to unsigned short.
  */
 template<bool indexRequest>
 static unsigned int getIndexOrObjectSize (unsigned int size)
@@ -1581,6 +1582,7 @@ void Block::initEmptyBlock(TLSData *tls, size_t size)
     unsigned int objSz = getObjectSize(size);
 
     cleanBlockHeader();
+    MALLOC_ASSERT(objSz <= USHRT_MAX, "objSz must not be less 2^16-1");
     objectSize = objSz;
     markOwned(tls);
     // bump pointer should be prepared for first allocation - thus mode it down to objectSize