fix manual ride entry crash when no zones file

This is a partial patch based on code from Eric Murray.  It changes just
enough of ManyalRideDialog to fix a crash that otherwise occurs when the
user doesn't have a zones file.

src/ManualRideDialog.cpp

@@ -226,6 +226,7 @@ ManualRideDialog::bsEstChanged()
 void
 ManualRideDialog::setBsEst()
 {
+    if (estBSbyDistButton) {
         if (estBSbyDistButton->isChecked()) {
             estBSFromDistance();
         }
@@ -233,6 +234,7 @@ ManualRideDialog::setBsEst()
             estBSFromTime();
         }
     }
+}
 
 
 void