Fix bug with remidx

remidx could exceed the number of legend entries. For example:

```matlab
figure
ezplot sin
legend sin
legtools.remove(legend,2)
% 2 exceeds the number of legend entries, which is 1
```

This commit fixes this bug by throwing an error if the largest value in
remidx is greater than the number of legend entries.

Author: erikhuizinga

legtools.m

@@ -133,6 +133,12 @@ function remove(lh, remidx)
                 'number of legend entries.'] ...
                 )
 
+            assert( ...
+                max(remidx) <= numel(lh.String), ...
+                'legtools:remove:BadSubscript', ...
+                'Index in remidx exceeds number of legend entries.' ...
+                )
+            
             if numel(unique(remidx)) == numel(lh.String)
                 delete(lh);
                 warning('legtools:remove:LegendDeleted', ...