Fix Visual C++ compiler warnings

Somehow this got reverted with aa769febf25c64f115c2a237516b0c7d65f651cd.
Oops.
diff --git a/rdppm.c b/rdppm.c
index f496ab3..a249142 100644
--- a/rdppm.c
+++ b/rdppm.c
@@ -99,7 +99,8 @@
 /* Note that on a 16-bit-int machine, only values up to 64k can be read. */
 /* This should not be a problem in practice. */
 {
-  register int ch, val;
+  register int ch;
+  register unsigned int val;
 
   /* Skip any leading whitespace */
   do {