hints: Fix leak with XResQueryClientIds

Leak report

8 bytes in 2 blocks are definitely lost in loss record 423 of 9,791
   at 0x48447A8: malloc (vg_replace_malloc.c:446)
   by 0x49A9ACE: UnknownInlinedFun (XRes.c:244)
   by 0x49A9ACE: XResQueryClientIds (XRes.c:286)
   by 0x135167: getWindowPID (hints.c:1352)
   by 0x11E507: clientFrame (client.c:1829)
   by 0x12D0CC: handleMapRequest (events.c:1171)
   by 0x12D0CC: handleEvent (events.c:2221)
   by 0x12D0CC: xfwm4_event_filter (events.c:2302)
   by 0x12A651: eventXfwmFilter (event_filter.c:175)
[...snip...]

Reference

• https://gitlab.freedesktop.org/xorg/lib/libxres/-/blob/bed46b55c06a3488971fda8271cf9fc0e3dd84d5/src/XRes.c#L295

Testing

• No issues were detected when testing the patch with Valgrind