Fix oob reads when fgets returns "\0". (closes #3776)

Fix oob reads when fgets returns "\0".  (closes #3776)

The ticket reported an out of bounds read in mutt_read_rfc822_line()
when a '\0' was embedded on its own line in the headers.  The function
assumed if fgets() didn't return NULL, then the string would have at
least one character.

I scanned the rest of the code and found three other places making the
same assumption for fgets.

Thanks to hanno for finding this with the "american fuzzy lop" tool.