b58enc: Make loop variables unsigned

zcount can only ever be positive as it starts from 0 and is incremented.
i can only be positive as it starts from zcount and is incremented. This
removes one of three sign compare warnings in this function.

base58.c

@@ -146,8 +146,8 @@ bool b58enc(char *b58, size_t *b58sz, const void *data, size_t binsz)
 {
 	const uint8_t *bin = data;
 	int carry;
-	ssize_t i, j, high, zcount = 0;
-	size_t size;
+	ssize_t j, high;
+	size_t i, size, zcount = 0;
 	
 	while (zcount < binsz && !bin[zcount])
 		++zcount;