client: eliminate destructive head parsing

While parsing the head of an MGCP response the r->body buffer is
manipulated in order to NUL terminate the extracted comment filed.

- Use a static buffer to store and manipulate the comment field.

Change-Id: Ib273c13d6fe7ee042fb4e3b8ed46ac02602226f6
diff --git a/include/osmocom/mgcp_client/mgcp_client.h b/include/osmocom/mgcp_client/mgcp_client.h
index 676850f..73f3bba 100644
--- a/include/osmocom/mgcp_client/mgcp_client.h
+++ b/include/osmocom/mgcp_client/mgcp_client.h
@@ -29,7 +29,7 @@
 struct mgcp_response_head {
 	int response_code;
 	mgcp_trans_id_t trans_id;
-	const char *comment;
+	char comment[MGCP_COMMENT_MAXLEN];
 	char conn_id[MGCP_CONN_ID_LENGTH];
 	char endpoint[MGCP_ENDPOINT_MAXLEN];
 };