rtp_engine: Prevent possible double free with DTLS config

ASTERISK-27225 #close
Reported by: Richard Kenner

Change-Id: I097b81734ef730f8603c0b972909d212a3a5cf89

main/rtp_engine.c

@@ -2702,35 +2702,35 @@ int ast_rtp_dtls_cfg_parse(struct ast_rtp_dtls_cfg *dtls_cfg, const char *name,
 			return -1;
 		}
 	} else if (!strcasecmp(name, "dtlscertfile")) {
-		ast_free(dtls_cfg->certfile);
 		if (!ast_strlen_zero(value) && !ast_file_is_readable(value)) {
 			ast_log(LOG_ERROR, "%s file %s does not exist or is not readable\n", name, value);
 			return -1;
 		}
+		ast_free(dtls_cfg->certfile);
 		dtls_cfg->certfile = ast_strdup(value);
 	} else if (!strcasecmp(name, "dtlsprivatekey")) {
-		ast_free(dtls_cfg->pvtfile);
 		if (!ast_strlen_zero(value) && !ast_file_is_readable(value)) {
 			ast_log(LOG_ERROR, "%s file %s does not exist or is not readable\n", name, value);
 			return -1;
 		}
+		ast_free(dtls_cfg->pvtfile);
 		dtls_cfg->pvtfile = ast_strdup(value);
 	} else if (!strcasecmp(name, "dtlscipher")) {
 		ast_free(dtls_cfg->cipher);
 		dtls_cfg->cipher = ast_strdup(value);
 	} else if (!strcasecmp(name, "dtlscafile")) {
-		ast_free(dtls_cfg->cafile);
 		if (!ast_strlen_zero(value) && !ast_file_is_readable(value)) {
 			ast_log(LOG_ERROR, "%s file %s does not exist or is not readable\n", name, value);
 			return -1;
 		}
+		ast_free(dtls_cfg->cafile);
 		dtls_cfg->cafile = ast_strdup(value);
 	} else if (!strcasecmp(name, "dtlscapath") || !strcasecmp(name, "dtlscadir")) {
-		ast_free(dtls_cfg->capath);
 		if (!ast_strlen_zero(value) && !ast_file_is_readable(value)) {
 			ast_log(LOG_ERROR, "%s file %s does not exist or is not readable\n", name, value);
 			return -1;
 		}
+		ast_free(dtls_cfg->capath);
 		dtls_cfg->capath = ast_strdup(value);
 	} else if (!strcasecmp(name, "dtlssetup")) {
 		if (!strcasecmp(value, "active")) {