Bad config stanza, balsa segv

[Geoffrey Leach <geoff hughes net>]

This happens with both the F12 version (balsa-2.4.1-2.fc12.i686) and a 
locally-built 2.4.2.  I have no idea what caused this stanza with an 
empty viewByUrl, but a check seems to be called for.

[viewByUrl-]
GUIFilter=0
SortField=1
Show=2
Open=true
Unread=0
Total=1
ModTime=1263938960