fuzz.diff

Block realloc_block(Block bk, uint32_t size) {
   assert(!memcmp(bk.memptr, bk.data, bk.data_size));
-  bk.data = randbytes(size);
-  bk.memptr = nrealloc(bk.memptr, size);
+  uint32_t preserved_size = bk.data_size < size ? bk.data_size : size;
+  char *new_memptr = nrealloc(bk.memptr, size);
+  assert(new_memptr != NULL);
+  assert(!memcmp(new_memptr, bk.data, preserved_size));
+
+  free(bk.data);
+  bk.data = randbytes(size);
+  bk.memptr = new_memptr;
   memcpy(bk.memptr, bk.data, size);
   bk.data_size = size;
   assert(!memcmp(bk.memptr, bk.data, bk.data_size));
   return bk;
 }