Don't compare changeset bbox coordinates when they may be nil

The code assumed that the changeset bounding box would only be
updated with a valid bounding box, but this isn't the case when
dealing with certain corner cases, including when removing
deleted members from relations.

app/models/changeset.rb

@@ -115,6 +115,10 @@ class Changeset < ActiveRecord::Base
     # nils, just use the bounding box update to write over them.
     @bbox = bbox.zip(array).collect { |a, b| a.nil? ? b : a }
 
+    # only try to update the bbox if there is a value for every coordinate
+    # which there will be from the previous line as long as both array and
+    # bbox are all non-nil. 
+    if has_valid_bbox? and array.all?
       # FIXME - this looks nasty and violates DRY... is there any prettier
       # way to do this?
       @bbox[0] = [-180 * GeoRecord::SCALE, array[0] + EXPAND * (@bbox[0] - @bbox[2])].max if array[0] < @bbox[0]
@@ -126,6 +130,7 @@ class Changeset < ActiveRecord::Base
       # whether this object needs saving or not.
       self.min_lon, self.min_lat, self.max_lon, self.max_lat = @bbox
     end
+  end
 
   ##
   # the number of elements is also passed in so that we can ensure that