Fix implicit type conversions and warnings on windows builds

The visual studio windows builds spit dozens of lines of warnings
on these.

Signed-off-by: Jakub Jelen <jjelen@redhat.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>

src/base64.c

@@ -183,7 +183,7 @@ error:
 static int to_block4(unsigned long *block, const char *source, int num)
 {
     const char *ptr = NULL;
-    unsigned int i;
+    size_t i;
 
     *block = 0;
     if (num < 1) {