Comment 2 for bug 1640457

StacktraceTop:
 __assert_fail_base (fmt=<optimized out>, assertion=assertion@entry=0x7fd94a537728 "bkref", file=file@entry=0x7fd94a53770a "../../nouveau/pushbuf.c", line=line@entry=238, function=function@entry=0x7fd94a537768 <__PRETTY_FUNCTION__.6215> "pushbuf_krel") at assert.c:92
 __GI___assert_fail (assertion=0x7fd94a537728 "bkref", file=0x7fd94a53770a "../../nouveau/pushbuf.c", line=238, function=0x7fd94a537768 <__PRETTY_FUNCTION__.6215> "pushbuf_krel") at assert.c:101
 pushbuf_krel () from /tmp/apport_sandbox_HhoZdt/usr/lib/x86_64-linux-gnu/libdrm_nouveau.so.2
 nouveau_pushbuf_reloc () from /tmp/apport_sandbox_HhoZdt/usr/lib/x86_64-linux-gnu/libdrm_nouveau.so.2
 PUSH_RELOC (tor=0, vor=0, flags=4096, offset=0, bo=<optimized out>, push=0x563469a72df0) at ../../../../../src/gallium/drivers/nouveau/nv30/nv30_winsys.h:26