Fix accesses to first element of an empty string.

Thanks to Olaf van der Spek for the report.


[SVN r33778]

libs/program_options

@@ -1 +1 @@
-Subproject commit 026c527d8d459f69b84557d08dd2d70d824e167b
+Subproject commit 3765e8e8e948c4b9f463f8353a0df0c08ebeb8cf