Turns out the DBUG_ASSERT added by fix for Bug#11792200 was overly pessimistic:
'stop0' is used in the main loop of do_div_mod, but we only dereference 'buf0'
for div operations, not for mod.


mysql-test/r/func_math.result:
  New test case.
mysql-test/t/func_math.test:
  New test case.
strings/decimal.c:
  Move DBUG_ASSERT down to where we actually dereference the loop pointer.