shm: Zero busy count for allocated buffers

This avoids a valgrind error like:

==31496== Conditional jump or move depends on uninitialised value(s)
==31496==    at 0x407620: weston_buffer_post_release (compositor.c:928)
==31496==    by 0x406AEB: weston_surface_attach (compositor.c:725)
==31496==    by 0x409EB8: pointer_attach (compositor.c:2009)
==31496==    by 0x34ECE05D63: ffi_call_unix64 (unix64.S:75)
==31496==    by 0x34ECE05784: ffi_call (ffi64.c:486)
==31496==    by 0x5674C4D: wl_closure_invoke (connection.c:770)
==31496==    by 0x566ECCB: wl_client_connection_data (wayland-server.c:255)
==31496==    by 0x56722F9: wl_event_source_fd_dispatch (event-loop.c:79)
==31496==    by 0x5672C99: wl_event_loop_dispatch (event-loop.c:410)
==31496==    by 0x56705FF: wl_display_run (wayland-server.c:1004)
==31496==    by 0x40C775: main (compositor.c:2937)
==31496==  Uninitialised value was created by a heap allocation
==31496==    at 0x4A074CD: malloc (vg_replace_malloc.c:236)
==31496==    by 0x5670EA7: shm_pool_create_buffer (wayland-shm.c:113)
==31496==    by 0x34ECE05D63: ffi_call_unix64 (unix64.S:75)
==31496==    by 0x34ECE05784: ffi_call (ffi64.c:486)
==31496==    by 0x5674C4D: wl_closure_invoke (connection.c:770)
==31496==    by 0x566ECCB: wl_client_connection_data (wayland-server.c:255)
==31496==    by 0x56722F9: wl_event_source_fd_dispatch (event-loop.c:79)
==31496==    by 0x5672C99: wl_event_loop_dispatch (event-loop.c:410)
==31496==    by 0x56705FF: wl_display_run (wayland-server.c:1004)
==31496==    by 0x40C775: main (compositor.c:2937)