MINOR: spoe: Set the argument chunk size to 0 when SPOE variables are checked

When SPOE variables are registered during HAProxy startup, the argument used to
call the function vars_check_arg() uses the trash area. To be sure it is never
released by the callee function, the size of the internal chunk (arg.data.str)
is set to 0. It is important to do so because, to fix a memory leak, this buffer
must be released by the function vars_check_arg().

This patch must be backported to 1.9.
diff --git a/src/flt_spoe.c b/src/flt_spoe.c
index 82e2719..2755f3a 100644
--- a/src/flt_spoe.c
+++ b/src/flt_spoe.c
@@ -4173,6 +4173,7 @@
 		arg.type = ARGT_STR;
 		arg.data.str.area = trash.area;
 		arg.data.str.data = trash.data;
+		arg.data.str.size = 0; /* Set it to 0 to not release it in vars_check_args() */
 		if (!vars_check_arg(&arg, err)) {
 			memprintf(err, "SPOE agent '%s': failed to register variable %s.%s (%s)",
 				  curagent->id, curagent->var_pfx, curagent->var_on_error, *err);
@@ -4189,6 +4190,7 @@
 		arg.type = ARGT_STR;
 		arg.data.str.area = trash.area;
 		arg.data.str.data = trash.data;
+		arg.data.str.size = 0;  /* Set it to 0 to not release it in vars_check_args() */
 		if (!vars_check_arg(&arg, err)) {
 			memprintf(err, "SPOE agent '%s': failed to register variable %s.%s (%s)",
 				  curagent->id, curagent->var_pfx, curagent->var_t_process, *err);
@@ -4205,6 +4207,7 @@
 		arg.type = ARGT_STR;
 		arg.data.str.area = trash.area;
 		arg.data.str.data = trash.data;
+		arg.data.str.size = 0;  /* Set it to 0 to not release it in vars_check_args() */
 		if (!vars_check_arg(&arg, err)) {
 			memprintf(err, "SPOE agent '%s': failed to register variable %s.%s (%s)",
 				  curagent->id, curagent->var_pfx, curagent->var_t_process, *err);
@@ -4410,6 +4413,7 @@
 		arg.type = ARGT_STR;
 		arg.data.str.area = trash.area;
 		arg.data.str.data = trash.data;
+		arg.data.str.size = 0;  /* Set it to 0 to not release it in vars_check_args() */
 		if (!vars_check_arg(&arg, err)) {
 			memprintf(err, "SPOE agent '%s': failed to register variable %s.%s (%s)",
 				  curagent->id, curagent->var_pfx, vph->name, *err);