Invalid table positions may also be too low.

git-svn-id: svn+ssh://src.earth.threerings.net/narya/trunk@3458 542714f4-19e9-0310-aa3c-eee0fc999fb1

src/java/com/threerings/parlor/data/Table.java

@@ -176,7 +176,7 @@ public class Table
         }
 
         // make sure the requested position is a valid one
-        if (position >= maxpos) {
+        if (position >= maxpos || position < 0) {
             return INVALID_TABLE_POSITION;
         }