simplify the check for out of workspace in URL encode

src/url.c

@@ -81,6 +81,7 @@ url_encode(const enum encoding enc, char *restrict const buf,
 	   const size_t inlen)
 {
 	char *p = buf;
+	const char * const end = buf + buflen;
 	const char *alphabet = hex_alphabet[0];
 
 	AN(buf);
@@ -93,12 +94,12 @@ url_encode(const enum encoding enc, char *restrict const buf,
 
 	for (int i = 0; i < inlen; i++) {
 		if (isunreserved(in[i])) {
-			if (p + 1 - buf > buflen)
+			if (p == end)
 				return -1;
 			*p++ = in[i];
 		}
 		else {
-			if (p + 3 - buf > buflen)
+			if (p + 3 > end)
 				return -1;
 			*p++ = '%';
 			*p++ = alphabet[(in[i] & 0xf0) >> 4];