always return something in GetCommandForVerb() second argument, even if verb not found

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@37940 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/unix/mimetype.cpp

@@ -124,6 +124,11 @@ public:
             if ( idx )
                 *idx = n;
         }
+        else if ( idx )
+        {
+            // different from any valid index
+            *idx = (size_t)-1;
+        }
 
         return s;
     }