Revert variable signedness changes.

In the line:

    for (carry = bin[i], j = size - 1; (j > high) || carry; --j)

my analysis missed that the compare (j > high) is always true when
high is 0 and j wraps around. So although the loop does not store
through a negative 'j' offset, it does require j to become negative
in order to terminate the loop in that case. My apologies.

base58.c

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