Lock panic buffer before mangling it

If we were panic-ing in multiple threads at the same time, the panic
buffer would be partially overwritten.  Prevent this with a mutex

bin/varnishd/cache_panic.c

@@ -34,6 +34,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <unistd.h>
+#include <pthread.h>
 
 #ifndef HAVE_EXECINFO_H
 #include "compat/execinfo.h"
@@ -56,6 +57,7 @@
  */
 
 static struct vsb vsps, *vsp;
+pthread_mutex_t panicstr_mtx = PTHREAD_MUTEX_INITIALIZER;
 
 /*--------------------------------------------------------------------*/
 
@@ -294,6 +296,9 @@ pan_ic(const char *func, const char *file, int line, const char *cond,
 	const char *q;
 	const struct sess *sp;
 
+	AZ(pthread_mutex_lock(&panicstr_mtx)); /* Won't be released,
+						  we're going to die
+						  anyway */
 	switch(xxx) {
 	case 3:
 		VSB_printf(vsp,

bin/varnishd/cache_shmlog.c

@@ -288,7 +288,7 @@ VSL_Init(void)
 
 	vsl_wrap();
 	VSM_head->starttime = (intmax_t)TIM_real();
-	VSM_head->panicstr[0] = '\0';
+	memset(VSM_head->panicstr, '\0', sizeof *VSM_head->panicstr);
 	memset(VSC_C_main, 0, sizeof *VSC_C_main);
 	VSM_head->child_pid = getpid();
 }