b58enc: Make loop variables unsigned

size can only be positive (and non-zero) as it is set to a positive expr + 1.
high is initialised with size-1 which is never negative, and then set to j at
the end of each loop iteration. It therefore can only be negative if j can.
The heart of the loop assigns to buf[j] which is on the stack. If j can become
negative during the loop then this is clearly undefined behaviour.

Following the loop, high is unused and j is reset to 0 and only incremented.
Thus both variables can be declared unsigned, and sys/types.h removed since
there are no more ssize_t variables remaining. This removes the last two sign
compare warnings from this file.

base58.c

@@ -15,7 +15,6 @@
 #include <stddef.h>
 #include <stdint.h>
 #include <string.h>
-#include <sys/types.h>
 
 #include "libbase58.h"
 
@@ -146,8 +145,7 @@ bool b58enc(char *b58, size_t *b58sz, const void *data, size_t binsz)
 {
 	const uint8_t *bin = data;
 	int carry;
-	ssize_t j, high;
-	size_t i, size, zcount = 0;
+	size_t i, j, size, high, zcount = 0;
 	
 	while (zcount < binsz && !bin[zcount])
 		++zcount;