Added field 'srclen' to structure 'lua_Debug'

This new field gets the length of 'source' in the same structure.
Unlike the other strings in that structure, 'source' can be
relatively large, and Lua already has its length readily available.

ldblib.c

@@ -167,7 +167,8 @@ static int db_getinfo (lua_State *L) {
     return luaL_argerror(L, arg+2, "invalid option");
   lua_newtable(L);  /* table to collect results */
   if (strchr(options, 'S')) {
-    settabss(L, "source", ar.source);
+    lua_pushlstring(L, ar.source, ar.srclen);
+    lua_setfield(L, -2, "source");
     settabss(L, "short_src", ar.short_src);
     settabsi(L, "linedefined", ar.linedefined);
     settabsi(L, "lastlinedefined", ar.lastlinedefined);