lynx-dev dev17: core dump with SOURCE_CACHE

[Leonid Pauzner]

Trying dev17 I run into core dump when using SOURCE_CACHE!=NONE
The problem came from completely removing 'from_source_cache' from
mainloop(), I will send a fix tomorrow. Crash can be seen when switching
'\'source or trying scrolling down in source mode. If you run into the
problem switch to SOURCE_CACHE:TRUE as a temporary workaround.

Leonid.