[CLI] bug in notmuch setup

[Patrick Totzke]

Hi all,

As mentioned on IRC earlier, current master seems to have an issue with `notmuch setup`,
which fails to write ~/.notmuch-config if not already present.

Here are excerpts from the discussion from #notmuch:
--------------------------------------%<---------------------------------------
19:08:29 <pazz> i have no notmuch config so far. when i run `notmuch` i get questioned about my name and so forth. 
                then:
19:08:31 <pazz> Error canonicalizing /home/pazz/.notmuch-config: No such file or directory
19:14:01 <bremner> it could also be a bug. There was some change recently to avoid overwriting symlinks with files.
19:15:29 <bremner> pazz: I think you found a bug. try "touch .notmuch-config" and run notmuch-setup again.
19:21:54 <pazz> bremner: i'm a little confused because i thought i'd done that already. anyway: notmuch setup 
                fails iff there is no ~/.notmuch-config present.
19:22:38 <pazz> if there is an empty file notmuch setup runs ok, exit code is 0 and the file gets overwritten
...
19:43:55 <Domo_> Error canonicalizing /home/too/.notmuch-config: No such file or directory
19:46:52 <Domo_> apparently realpath(3) fails in case there is no such file ;/
19:50:50 <pazz> forgot to mention: i'm on 0.15.2+85~g44fe1f2d
19:52:55 <bremner> yeah, the offending patch is +79, I think
19:53:25 <Domo_> the reason is obvious .. notmuch_config_save() executes     filename = realpath
                 (config->filename, NULL);
19:53:53 <Domo_> maybe just additionally checking for ENOENT could fix that...
19:57:20 <bremner> sounds plausible. Have to double check we are not relying on some implicit re-allocation by 
                   realpath.
19:58:41 <Domo_> true: free (filename) needs not to be done in case realpath() fails with ENOENT

-------------------------------------->%---------------------------------------

Best,
/p
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