Tell coverity to tolerate ed25519_verify

src/ext/ed25519/donna/ed25519-donna.h

@@ -68,6 +68,7 @@ ed25519_verify(const unsigned char *x, const unsigned char *y, size_t len) {
 	size_t differentbits = 0;
 	while (len--)
 		differentbits |= (*x++ ^ *y++);
+        /*coverity[overflow]*/
 	return (int) (1 & ((differentbits - 1) >> 8));
 }