common: Fix connecting to the agent.

* common/homedir.c (_gnupg_socketdir_internal): Fix error handling.

Prior to 26086b36 the non-existance of the socket directory was
considered an error if a non-default home directory is used. Since
26086b36 we now create the directory on demand, but the function still
returned the fallback path. This made the agent bind the socket in
the socket directory, and the client trying to connect to the socket
in the home directory.

• Fixes-commit: 26086b362ff47d21b1abefaf674a6464bf0a8921
• Signed-off-by: Justus Winter <justus@g10code.com>