BUG/MINOR: lua: error in detection of mandatory arguments

The last mandatory arguments in sample fetches or converters are
ignored. This is due to a bad control.
diff --git a/src/hlua.c b/src/hlua.c
index d1086c7..1c239e5 100644
--- a/src/hlua.c
+++ b/src/hlua.c
@@ -530,7 +530,7 @@
 
 		/* Check for mandatory arguments. */
 		if (argp[idx].type == ARGT_STOP) {
-			if (idx + 1 < min_arg)
+			if (idx < min_arg)
 				WILL_LJMP(luaL_argerror(L, first + idx, "Mandatory argument expected"));
 			return 0;
 		}