CLEANUP: hlua: simplify ambiguous lua_insert() usage in hlua_ctx_resume()

'lua_insert(lua->T, -lua_gettop(lua->T))' is actually used to rotate the
top value with the bottom one, thus the code was overkill and the comment
was actually misleading, let's fix that by using explicit equivalent form
(absolute index).

It may be backported with 5508db9 ("BUG/MINOR: hlua: fix unsafe
lua_tostring() usage with empty stack") to all stable versions to ease
code maintenance.

(cherry picked from commit 2bde0d6)
Signed-off-by: Amaury Denoyelle <[email protected]>

Author: Darlelet

src/hlua.c

@@ -2087,8 +2087,8 @@ static enum hlua_exec hlua_ctx_resume(struct hlua *lua, int yield_allowed)
 			hlua_pushfstring_safe(lua->T, "[state-id %d] unknown runtime error from %s",
 			                      lua->state_id, trace);
 
-		/* Move the error msg at the top and then empty the stack except last msg */
-		lua_insert(lua->T, -lua_gettop(lua->T));
+		/* Move the error msg at the bottom and then empty the stack except last msg */
+		lua_insert(lua->T, 1);
 		lua_settop(lua->T, 1);
 		ret = HLUA_E_ERRMSG;
 		break;
@@ -2113,8 +2113,8 @@ static enum hlua_exec hlua_ctx_resume(struct hlua *lua, int yield_allowed)
 			hlua_pushfstring_safe(lua->T, "[state-id %d] message handler error",
 			                      lua->state_id);
 
-		/* Move the error msg at the top and then empty the stack except last msg */
-		lua_insert(lua->T, -lua_gettop(lua->T));
+		/* Move the error msg at the bottom and then empty the stack except last msg */
+		lua_insert(lua->T, 1);
 		lua_settop(lua->T, 1);
 		ret = HLUA_E_ERRMSG;
 		break;