gesture.c: Properly free valuator mask

valuator_mask_free() takes a pointer to a pointer to a valuator mask,
because it sets its value to NULL after free'ing.

BUG=chromium-os:32657
TEST=tail -f /var/log/Xort.0.log
TEST=stop ui
TEST=No "Segmentation Fault" in /var/log/Xorg.0.log
TEST=Instead: "Server terminated successfully (0). Closing log file."

Change-Id: Ib01c0d0aafc5f92130d2005d6477315babb02f45
1 file changed