try to narrow down a umem panic observed in vtest b00035.vtc

is it a race with _close ?

***  v1   debug|Child (369) Panic at: Tue, 14 Jan 2020 12:06:12 GMT
***  v1   debug|Wrong turn at
../../../bin/varnishd/cache/cache_main.c:284:
***  v1   debug|Signal 11 (Segmentation Fault) received at b4 si_code 1
***  v1   debug|version = varnish-trunk revision
b8b798a0, vrt api = 10.0
***  v1   debug|ident = -jsolaris,-sdefault,-sdefault,-hcritbit,ports
***  v1   debug|now = 2786648.903965 (mono), 1579003571.310573 (real)
***  v1   debug|Backtrace:
***  v1   debug|  80e1bd8: /tmp/vtest.o32_su12.4/varnish-cache/varnish-trunk/_build/bin/varnishd/varnishd'pan_backtrace+0x18 [0x80e1bd8]
***  v1   debug|  80e2147: /tmp/vtest.o32_su12.4/varnish-cache/varnish-trunk/_build/bin/varnishd/varnishd'pan_ic+0x2c7 [0x80e2147]
***  v1   debug|  81b9a6f: /tmp/vtest.o32_su12.4/varnish-cache/varnish-trunk/_build/bin/varnishd/varnishd'VAS_Fail+0x4f [0x81b9a6f]
***  v1   debug|  80d7fba: /tmp/vtest.o32_su12.4/varnish-cache/varnish-trunk/_build/bin/varnishd/varnishd'child_signal_handler+0x27a [0x80d7fba]
***  v1   debug|  fed92695: /lib/libc.so.1'__sighndlr+0x15 [0xfed92695]
***  v1   debug|  fed86c8b: /lib/libc.so.1'call_user_handler+0x298 [0xfed86c8b]
***  v1   debug|  fda8a93e: /lib/libumem.so.1'umem_cache_free
***  v1   debug|+0x23 [0xfda8a93e]
***  v1   debug|  817f3bc: /tmp/vtest.o32_su12.4/varnish-cache/varnish-trunk/_build/bin/varnishd/varnishd'smu_free+0x35c [0x817f3bc]
***  v1   debug|  817aa21: /tmp/vtest.o32_su12.4/varnish-cache/varnish-trunk/_build/bin/varnishd/varnishd'sml_stv_free+0x101 [0x817aa21]
***  v1   debug|  817b4eb: /tmp/vtest.o32_su12.4/varnish-cache/varnish-trunk/_build/bin/varnishd/varnishd'sml_slim+0x2cb [0x817b4eb]
***  v1   debug|thread = (cache-exp)
***  v1   debug|thr.req = 0 {
***  v1   debug|},
***  v1   debug|thr.busyobj = 0 {
***  v1   debug|},
***  v1   debug|vmods = {
***  v1   debug|},
***  v1   debug|
***  v1   debug|
***  v1   debug|Info: Child (369) said Child dies
***  v1   debug|Debug:
***  v1   debug| Child cleanup complete
***  v1   debug|

bin/varnishd/storage/storage_umem.c

@@ -223,6 +223,7 @@ smu_free(struct storage *s)
 
 	umem_freef(smu->s.ptr, smu->sz);
 	smu_smu_init(smu, sc);
+	AN(sc->smu_cache);
 	umem_cache_freef(sc->smu_cache, smu);
 }