scm_boot_guile: Gracefully handle the case where argc == 0.

Fixes <http://bugs.gnu.org/18680>.
Reported by Nala Ginrut <nalaginrut@gmail.com>.

* libguile/init.c (scm_boot_guile): Do not canonicalize argv[0] unless
  argc > 0.
* THANKS: Add Nala Ginrut to the fixes section.

THANKS

@@ -86,6 +86,7 @@ For fixes or providing information which led to a fix:
          Andrew Gaylard
            Nils Gey
            Eric Gillespie, Jr
+           Nala Ginrut
          Didier Godefroy
   Panicz Maciej Godek
            John Goerzen