dix: reset the registry before quitting

dix: reset the registry before quitting

Heaps of these:
==2042== 15,360 bytes in 120 blocks are still reachable in loss record 94 of
97
==2042==    at 0x4C2A4CD: malloc (vg_replace_malloc.c:236)
==2042==    by 0x4C2A657: realloc (vg_replace_malloc.c:525)
==2042==    by 0x45FB91: double_size (registry.c:65)
==2042==    by 0x45FC97: RegisterRequestName (registry.c:85)
==2042==    by 0x460095: RegisterExtensionNames (registry.c:179)
==2042==    by 0x460729: dixResetRegistry (registry.c:334)
==2042==    by 0x5AC992: main (main.c:201)

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Keith Packard <keithp@keithp.com>