scpi-dmm: free memory that was allocated by SCPI get routines

The SCPI get routines may allocate memory for response data which
callers have to free after use.

This addresses part of bug #1683.

src/hardware/scpi-dmm/protocol.c

@@ -100,8 +100,10 @@ SR_PRIV int scpi_dmm_get_mq(const struct sr_dev_inst *sdi,
 	ret = sr_scpi_get_string(sdi->conn, command, &response);
 	if (ret != SR_OK)
 		return ret;
-	if (!response || !*response)
+	if (!response || !*response) {
+		g_free(response);
 		return SR_ERR_NA;
+	}
 	have = response;
 	if (*have == '"')
 		have++;