MINOR: hlua: don't dump empty entries in hlua_traceback()

Calling hlua_traceback() sometimes reports empty entries looking like:

   [C]: ?

These ones correspond to certain internal C functions of the Lua library,
but they do not provide any information and even complicate the
interpretation of the dump. Better just skip them.

src/hlua.c

@@ -505,11 +505,6 @@ __LJMP const char *hlua_traceback(lua_State *L, const char* sep)
 	struct buffer *msg = get_trash_chunk();
 
 	while (lua_getstack(L, level++, &ar)) {
-
-		/* Add separator */
-		if (b_data(msg))
-			chunk_appendf(msg, "%s", sep);
-
 		/* Fill fields:
 		 * 'S': fills in the fields source, short_src, linedefined, lastlinedefined, and what;
 		 * 'l': fills in the field currentline;
@@ -518,6 +513,14 @@ __LJMP const char *hlua_traceback(lua_State *L, const char* sep)
 		 */
 		lua_getinfo(L, "Slnt", &ar);
 
+		/* skip these empty entries, usually they come from deep C functions */
+		if (ar.currentline < 0 && *ar.what == 'C' && !*ar.namewhat && !ar.name)
+			continue;
+
+		/* Add separator */
+		if (b_data(msg))
+			chunk_appendf(msg, "%s", sep);
+
 		/* Append code localisation */
 		if (ar.currentline > 0)
 			chunk_appendf(msg, "%s:%d: ", ar.short_src, ar.currentline);