Crash on painting

[Sven Langkamp]

Hi,

currently Krita crashes sometimes on painting. Especially when you're clicking 
very fast. Looks like a problem with KisPaintDevice in KisTileMgr::bitBlt

gdb ouput:

Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 16384 (LWP 28412)]
0x4206ff6a in KisPainter::bitBlt(int, int, CompositeOp, 
KSharedPtr<KisPaintDevice>, unsigned char, int, int, int, int) 
(this=0x88cf880, dx=257, dy=272,
    op=COMPOSITE_NORMAL, srcdev={ptr = 0x0}, opacity=255 'ÿ', sx=0, sy=0,
    sw=-1, sh=-1) at kis_painter.cc:183
183             KisTileMgrSP srctm = srcdev -> data();