dispatcher: Fix rare crash with 'pvar_algo_pattern'

This patch simply moves the name buffer for the 'pvar_algo_pattern' into
SHM rather than a stack buffer, in order to avoid invalid memory being
referenced past the function's return point.

Many thanks to Eric Tamme from Five9 for reporting & testing!

(cherry picked from commit fe1a50d)

modules/dispatcher/dispatch.c

@@ -571,9 +571,16 @@ ds_pvar_param_p ds_get_pvar_param(int id, str uri)
 	int len = ds_pattern_prefix.len + ds_pattern_infix.len + ds_pattern_suffix.len 
 				+ uri.len + str_id.len;
 
-	char buf[len]; /* XXX: check if this works for all compilers */
+	char *buf;
 	ds_pvar_param_p param;
 
+	param = shm_malloc(sizeof *param + len);
+	if (!param) {
+		LM_ERR("no more shm memory\n");
+		return NULL;
+	}
+	buf = param->buf;
+
 	if (ds_pattern_one>DS_PATTERN_NONE) {
 		name.len = 0;
 		name.s = buf;
@@ -599,12 +606,6 @@ ds_pvar_param_p ds_get_pvar_param(int id, str uri)
 		name.len += ds_pattern_suffix.len;
 	}
 
-	param = shm_malloc(sizeof(ds_pvar_param_t));
-	if (!param) {
-		LM_ERR("no more shm memory\n");
-		return NULL;
-	}
-
 	if (!pv_parse_spec(ds_pattern_one>DS_PATTERN_NONE ? &name : &ds_pattern_prefix,
 	&param->pvar)) {
 		LM_ERR("cannot parse pattern spec\n");

modules/dispatcher/dispatch.h

@@ -107,6 +107,7 @@ typedef struct _ds_pvar_param
 {
 	pv_spec_t pvar;
 	int value;
+	char buf[0];
 } ds_pvar_param_t, *ds_pvar_param_p;