The failing assertion was written with the assumption that a NULL
string can never be passed to my_strtod(). However, an empty string
may be passed under some circumstances by passing str == NULL and
*end == NULL.

Fixed the assertion to take the above case into account.

mysql-test/r/func_misc.result:
  Added a test case for bug #52165.
mysql-test/t/func_misc.test:
  Added a test case for bug #52165.
strings/dtoa.c:
  Fixed the assertion in my_strtod() to take the case of 'str == NULL
  && *end == NULL' into account.