Segfaults if variable is set to ""

[Jonas Jacobsson <mutt@xxxxxxxxx>]

I have added a new index format variable that is configurable via
muttrc. But I want hdr_format_str() in hdrline.c to return an empty
string if the configured value is an empty string. But Mutt segfaults
when it is sorting it's mailbox (or is about to display the first
mail) if the variable is set to "". It seems that the segfault is
before it comes to the select/case statements.

If I set index_format to "" it works ok (i.e. doesn't segfaults).

Any clues?

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