sysmalloc: Initialize previous size field of mmaped chunks

With different encodings of the header, the previous zero initialization
may be insufficient and produce an invalid encoding.

malloc/malloc.c

@@ -2306,6 +2306,7 @@ sysmalloc (INTERNAL_SIZE_T nb, mstate av)
               else
                 {
                   p = (mchunkptr) mm;
+		  set_prev_size (p, 0);
                   set_head (p, size | IS_MMAPPED);
                 }