memory leak due to dict_set_filter() ref/unref

[Ben Pfaff]

Hi John!

Commit d01a4986ffc8 ("Fix crash in gui, when T-TEST or ONEWAY while a
filter is set.") makes dict_set_filter() ref the dict when setting a
filter.  This causes memory leaks in any test that sets a filter and
does not later explicitly clear the filter:

 318: save.at:47         SAVE -- delete unselected
 320: save-translate.at:48 CSV output -- recode missing, delete unselected
 321: save-translate.at:56 CSV output -- var names, formats
 323: save-translate.at:77 CSV output -- custom delimiter, qualifier
 324: save-translate.at:88 CSV output -- KEEP, RENAME
 328: delete-variables.at:20 DELETE VARIABLES with FILTER
 355: split-file.at:77   SPLIT FILE  - vs procedures
 684: regression.at:19   LINEAR REGRESSION - basic
 729: sort-cases.at:100  SORT CASES preserves filtered cases
 791: select-if.at:19    FILTER

I imagine there must be some way to fix the crash in the GUI without
adding memory leaks.  Can you take a look?

Thanks,

Ben.