cygheap_pwdgrp: Handle invalid db_* entries correctly

If the first scheme in db_* was invalid, the code would think there
were no schemes specified and replace the second scheme with
NSS_SCHEME_DESC.

Signed-off-by: David Macek <david.macek.0@gmail.com>

winsup/cygwin/uinfo.cc

@@ -823,7 +823,10 @@ cygheap_pwdgrp::nss_init_line (const char *line)
 					  c, e - c);
 		    }
 		  else
+		    {
 		      debug_printf ("Invalid nsswitch.conf content: %s", line);
+		      --idx;
+		    }
 		  c += strcspn (c, " \t");
 		  c += strspn (c, " \t");
 		  ++idx;