lib/param: handle (ignore) substitution variable in smb.conf

[github at samba.org]

New comment by obnoxxx on Samba Github repository

https://github.com/samba-team/samba/pull/12#issuecomment-151971578
Comment:
Generally, the patch looks good, as previously discussed. (Sorry for the delay...)

But a few (cosmetic comments regarding the patch):

* The else branch is not necessary, due to the return.
  ==> Please reduce indentation by removing the else (and moving the variable def's up)

* Please use "if (substitution_variable_substring != NULL)" 
   instead of " if (substitution_variable_substring) "

* Please explain a bit more about the problem your patch is solving in the commit message
  and include a line
  "BUG: https://bugzilla.samba.org/show_bug.cgi?id=10722"

With these changes. I'll be good.

Thanks - Michael