composite: initialize border clip even when pixmap alloc fails

If it fails to allocate the pixmap, the function compAllocPixmap() would
return early and leave the borderClip region uninitialized, which may
lead to the use of uninitialized value as reported by valgrind:

 Conditional jump or move depends on uninitialised value(s)
    at 0x4F9B33: compClipNotify (compwindow.c:317)
    by 0x484FC9: miComputeClips (mivaltree.c:476)
    by 0x48559A: miValidateTree (mivaltree.c:679)
    by 0x4F0685: MapWindow (window.c:2693)
    by 0x4A344A: ProcMapWindow (dispatch.c:922)
    by 0x4A25B5: Dispatch (dispatch.c:560)
    by 0x4B082A: dix_main (main.c:282)
    by 0x429233: main (stubmain.c:34)
  Uninitialised value was created by a heap allocation
    at 0x4841866: malloc (vg_replace_malloc.c:446)
    by 0x4F47BC: compRedirectWindow (compalloc.c:171)
    by 0x4FA8AD: compCreateWindow (compwindow.c:592)
    by 0x4EBB89: CreateWindow (window.c:925)
    by 0x4A2E6E: ProcCreateWindow (dispatch.c:768)
    by 0x4A25B5: D...