panic: neg writecount increment -1

Title	Repro	Cause bisect	Fix bisect	Count	Last	Reported
panic: vm_object_vndeallocate: bad object reference count	C			974	1808d	1809d

panic: neg writecount increment -1
cpuid = 1
time = 1557785065
KDB: stack backtrace:
db_trace_self_wrapper() at db_trace_self_wrapper+0x47/frame 0xfffffe00212fe4f0
vpanic() at vpanic+0x1e0/frame 0xfffffe00212fe550
panic() at panic+0x43/frame 0xfffffe00212fe5b0
vop_stdadd_writecount() at vop_stdadd_writecount+0xcd/frame 0xfffffe00212fe5f0
VOP_ADD_WRITECOUNT_APV() at VOP_ADD_WRITECOUNT_APV+0xc5/frame 0xfffffe00212fe620
vn_close1() at vn_close1+0x125/frame 0xfffffe00212fe6a0
vn_closefile() at vn_closefile+0x87/frame 0xfffffe00212fe730
_fdrop() at _fdrop+0x3a/frame 0xfffffe00212fe760
closef() at closef+0x27d/frame 0xfffffe00212fe7f0
fdescfree_fds() at fdescfree_fds+0xbd/frame 0xfffffe00212fe840
fdescfree() at fdescfree+0x58a/frame 0xfffffe00212fe900
exit1() at exit1+0x780/frame 0xfffffe00212fe970
sys_sys_exit() at sys_sys_exit+0xd/frame 0xfffffe00212fe980
amd64_syscall() at amd64_syscall+0x436/frame 0xfffffe00212feab0
fast_syscall_common() at fast_syscall_common+0x101/frame 0xfffffe00212feab0
--- syscall (1, FreeBSD ELF64, sys_sys_exit), rip = 0x44f54a, rsp = 0x7fffffffec58, rbp = 0x1 ---
KDB: enter: panic
[ thread pid 1286 tid 100700 ]
Stopped at      kdb_enter+0x6a: movq    $0,kdb_why