simplewallet: Use default log file name when executable's file path is unknown

Default to "simplewallet.log" in current directory when file path isn't
obtained from epee.

In this situation previously, it defaulted to the file name of ".log"
("" + ".log") in the current directory.
(Thanks to @sammy007 for reporting bug.)

An earlier version yet used "" + "/" + ".log" = "/.log", which resulted
in silently not logging in most cases, due to lack of permission.

Test:
  PATH=$PATH:</path/to/simplewallet/folder> && simplewallet --wallet-file /dev/null

This results in epee not finding the executable's file path, so
simplewallet will now use a default log filename.

0 files changed, 0 insertions, 0 deletions