hex: Simplify hex_encode

The documentation for hex_encode indicates that it returns simply true or
false. The old implementation was returning the written length on success,
cast to boolean. This will elicit a warning under MSVC.

On further examination, there is no need to check/modify the length inside
the loop, since we can check it once before starting. As a result the code
can be simplified a bit.

A side affect of this change is that nothing will be written at all if the
length is incorrect, vs the previous code writing characters until the length
available is exhausted. I prefer the new semantics but YMMV.
Signed-off-by: Jon Griffiths <jon_p_griffiths@yahoo.com>

ccan/str/hex/hex.c

@@ -50,21 +50,17 @@ static char hexchar(unsigned int val)
 
 bool hex_encode(const void *buf, size_t bufsize, char *dest, size_t destsize)
 {
-	size_t used = 0;
+	size_t i;
 
-	if (destsize < 1)
+	if (destsize < hex_str_size(bufsize))
 		return false;
 
-	while (used < bufsize) {
-		unsigned int c = ((const unsigned char *)buf)[used];
-		if (destsize < 3)
-			return false;
+	for (i = 0; i < bufsize; i++) {
+		unsigned int c = ((const unsigned char *)buf)[i];
 		*(dest++) = hexchar(c >> 4);
 		*(dest++) = hexchar(c & 0xF);
-		used++;
-		destsize -= 2;
 	}
 	*dest = '\0';
 
-	return used + 1;
+	return true;
 }