MDEV-26139 Spider crashes with segmentation fault (signal 11) on CREATE TABLE when COMMENT does not contain embedded double quotes

The root cause of the bug MDEV-26139 is the lack of NULL checking
on the variable `dq`.

Comments on if (dq && (!sq || sq > dq)) {...} else {...}:

  * The if block corresponds to the case where parameters are
    quoted by double quotes. In that case, a single quote doesn't
    appear at all or only appears in the middle of double quotes.

  * The else block corresponds to the case where parameters are
    quoted by single quotes. In that case, a double quote doesn't
    appear at all or only appears in the middle of single quotes.

  * If the program reaches the if-else statement, `sq || dq` holds.
    Thus, the negation of `dq && (!sq || sq > dq)` is equivalent to
    `sq && (!dq || sq <= dq)`.