acc: Fix possible crash if 'aaa' module not loaded

If do_accounting("aaa") is used without a backend module loaded,
OpenSIPS would actually start, then crash when generating an acc record.

Credits to Simon Gajski and Bogdan Iancu for reporting and diagnosing
the issue here.

modules/acc/acc.c

@@ -97,7 +97,7 @@ event_id_t acc_cdr_event = EVI_ERROR;
 event_id_t acc_event = EVI_ERROR;
 event_id_t acc_missed_event = EVI_ERROR;
 
-static db_func_t acc_dbf;
+db_func_t acc_dbf;
 static db_con_t* db_handle=0;
 extern int acc_log_facility;
 
@@ -850,6 +850,12 @@ int acc_aaa_request( struct sip_msg *req, struct sip_msg *rpl)
 	struct acc_extra* extra;
 	acc_ctx_t* ctx = try_fetch_ctx();
 
+	if (!proto.create_aaa_message) {
+		LM_BUG("failed to generate AAA record ('aaa' accounting was enabled"
+		        ", but no 'aaa' module is available!)");
+		return -1;
+	}
+
 	if ((send = proto.create_aaa_message(conn, AAA_ACCT)) == NULL) {
 		LM_ERR("failed to create new aaa message for acct\n");
 		return -1;
@@ -949,6 +955,12 @@ int acc_aaa_cdrs(struct dlg_cell *dlg, struct sip_msg *msg, acc_ctx_t* ctx)
 
 	struct acc_extra* extra;
 
+	if (!proto.create_aaa_message) {
+		LM_BUG("failed to generate AAA record ('aaa' accounting was enabled"
+		        ", but no 'aaa' module is available!)");
+		return -1;
+	}
+
 	core_s.s = extra_s.s = leg_s.s = 0;
 
 	ret = prebuild_core_arr(dlg, &core_s, &start_time);

modules/acc/acc.h

@@ -73,6 +73,7 @@
 #include "acc_logic.h"
 
 extern struct dlg_binds dlg_api;
+extern db_func_t acc_dbf;
 
 
 int store_extra_values(extra_value_t* values, str *values_str,

modules/acc/acc_logic.c

@@ -957,7 +957,7 @@ static void acc_cdr_cb( struct cell* t, int type, struct tmcb_params *ps )
 	}
 
 	if (is_aaa_acc_on(ctx->flags) && acc_aaa_cdrs(dlg, ps->req, ctx) < 0) {
-		LM_ERR("Cannot create radius accounting\n");
+		LM_ERR("Cannot perform radius accounting\n");
 		return;
 	}
 
@@ -1037,9 +1037,20 @@ unsigned long long do_acc_type_parser(str* token)
 		return DO_ACC_LOG;
 	} else if (token->len == do_acc_aaa_s.len &&
 			!strncasecmp(token->s, do_acc_aaa_s.s, token->len)) {
+		if (!proto.create_aaa_message) {
+			LM_ERR("do_accounting(\"aaa\") was used"
+			        ", but no 'aaa' module is loaded!\n");
+			return DO_ACC_ERR;
+		}
+
 		return DO_ACC_AAA;
 	} else if (token->len == do_acc_db_s.len &&
 			!strncasecmp(token->s, do_acc_db_s.s, token->len)) {
+		if (!acc_dbf.init) {
+			LM_ERR("do_accounting(\"db\") was used"
+			        ", but no 'db' module is loaded!\n");
+			return DO_ACC_ERR;
+		}
 		return DO_ACC_DB;
 	}  else if (token->len == do_acc_evi_s.len &&
 			!strncasecmp(token->s, do_acc_evi_s.s, token->len)) {