Fix a memory leak in `freeClass()` exposed by valgrind's memcheck

In `msGrowClassLabels()` memory is assigned to
`class->labels[class->numlabels]` but was only freed to
`class->numstyles-1` in `freeClass()`.  This fix ensures the last
label is freed.
Revert "Fix a memory leak in `freeClass()` exposed by ... memcheck"

This reverts commit 4b90c2404f4b3209f491ab84780f735fba4a119d which as
discussed in issue #4685 is not a valid solution.
Ensure class labels are free'd in cases where label loading fails
Merge pull request #4685 from homme/grow-class-labels-leak-fix

Fix a memory leak on failed parsing in `freeClass()` (#4685)