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.
7c4d6f1dc6