svn r486, crash while handling f=f mail

Due my usual reading of gentoo-users mail list I've came up with a f=f
mail message that keeps crashing muttng all the time. 

It seems that if mail is f=f and if it in its body contains line

|> \n

...(that means quote level one sign, space, newline), muttng is
crashed. 

The problem is in rfc3676.c, on line 202 and 203. Mentioned line causes
curline_len + buf_len - buf_off to be equal 0 which causes curline
pointer to point to the NULL. In the next line (203) strcpy is trying to
copy to the NULL pointer which leads to SIGSEGV signal.

-- 
Daniel Vrcic