delete should update preview window

[Jeff Horwitz <jhorwitz75 yahoo com>]

[already submitted to balsa bug list]

In preview mode, deleting a message advaces the index list to the next
message, but the preview window goes blank.  this is painful during message
browsing.  The following patch just adds one line to balsa-index-page.c
that
fixes the problem.

http://www.monkey.org/~jhorwitz/balsa/balsa-1.0.1-previewdelete.diff