Comment 2 for bug 1784900

The reason for this bug is memory corruption in glibc's memory chunk header that is in front of some bitmap pixman is allocating and maintaining as image->bits.free_me. I set a memory watchpoint to this memory location and this code here triggered it and corrupted what seems to be a memory chunk size indicator, which upon free() causes print of 'invalid pointer' by glibc:

Thread 1 "qemu-system-x86" hit Hardware watchpoint 2: *0x7f6160361d88

Old value = 3145749
New value = 0
vga_draw_line8 (vga=vga@entry=0x556d68549b30, d=0x7f6160361d80 "", d@entry=0x7f61603615e0 "", addr=983528, width=<optimized out>)
    at /home/stefanb/tmp/qemu-tip/hw/display/vga-helpers.h:297
297 ((uint32_t *)d)[3] = palette[vga_read_byte(vga, addr + 3)];

(gdb) bt
#0 vga_draw_line8 (vga=vga@entry=0x556d68549b30, d=0x7f6160361d80 "", d@entry=0x7f61603615e0 "", addr=983528, width=<optimized out>)
    at /home/stefanb/tmp/qemu-tip/hw/display/vga-helpers.h:297
#1 0x0000556d659918ee in vga_draw_graphic (full_update=0, s=0x556d68549b30) at /home/stefanb/tmp/qemu-tip/hw/display/vga.c:1695
#2 vga_update_display (opaque=0x556d68549b30) at /home/stefanb/tmp/qemu-tip/hw/display/vga.c:1782
#3 0x0000556d65c0cd92 in vnc_refresh (dcl=0x556d683055a8) at ui/vnc.c:3046
#4 0x0000556d65bff702 in dpy_refresh (s=0x556d686be540) at ui/console.c:1658
#5 gui_update (opaque=0x556d686be540) at ui/console.c:205
#6 0x0000556d65d0deac in timerlist_run_timers (timer_list=0x556d66de0e00) at util/qemu-timer.c:536
#7 0x0000556d65d0e0f7 in qemu_clock_run_timers (type=QEMU_CLOCK_REALTIME) at util/qemu-timer.c:547
#8 qemu_clock_run_all_timers () at util/qemu-timer.c:674
#9 0x0000556d65d0e5d1 in main_loop_wait (nonblocking=<optimized out>) at util/main-loop.c:503
#10 0x0000556d65a5f2ee in main_loop () at vl.c:1865
#11 0x0000556d658ff166 in main (argc=<optimized out>, argv=<optimized out>, envp=<optimized out>) at vl.c:4643