[dlmalloc] Fix -Wnull-pointer-arithmetic

Arithmetic on a NULL pointer is undefined behaviour and could be used by
the compiler to optimize out the arithmetic. Use the integer expansion of
chunk2mem(0) instead to silence this warning:
`arithmetic on a null pointer treated as a cast from integer to pointer is a GNU extension [-Wnull-pointer-arithmetic]`

external/lib/heap/dlmalloc/dlmalloc.c

@@ -2784,7 +2784,7 @@ static int has_segment_link(mstate m, msegmentptr ss) {
   noncontiguous segments are added.
 */
 #define TOP_FOOT_SIZE\
-  (align_offset(chunk2mem(0))+pad_request(sizeof(struct malloc_segment))+MIN_CHUNK_SIZE)
+  (align_offset(/*chunk2mem(0)*/TWO_SIZE_T_SIZES)+pad_request(sizeof(struct malloc_segment))+MIN_CHUNK_SIZE)
 
 
 /* -------------------------------  Hooks -------------------------------- */