qemu segfaults on PNG screendump
Description of problem:
Attempting to produce a screendump via the monitor in the PNG format leads to a segmentation fault (but the screen dump is produced correctly).
Steps to reproduce:
1. Launch QEMU
2. Go to the monitoring screen ()
3. execute the command: `screendump /tmp/dump.png -f png`
4. observe the crash (segfault)