MAINT: remove unnecessary null checks before free

free is required to handle null in all versions of the C standard and
15+ year olds systems which might not comply to this basic requiremnent
likely can't run current scipy anyway.
Patch generated with semantic patch:

    @@
    expression E;
    @@

    - if (E != NULL) {
    -   free(E);
    - }
    + free(E);
Merge pull request #4181 from juliantaylor/free-null

MAINT: remove unnecessary null checks before free