	Avoid undefined behavior when char is signed.

diff -ur xpdfopen-0.86.orig/sendx.c xpdfopen-0.86/sendx.c
--- xpdfopen-0.86.orig/sendx.c	2012-01-28 01:39:39.000000000 +0100
+++ xpdfopen-0.86/sendx.c	2014-06-18 08:13:50.000000000 +0200
@@ -229,7 +229,7 @@
      */
     if (ks < 256)
     {
-        event.state = isupper((char)ks);
+        event.state = isupper((unsigned char)ks);
         switch (ks)
         {
           case 0x08:
