Fix validation of overlong UTF-8

It was too strict and Egyptian dicks didn't want to pass,
so we'll do it half-arsedly for a subset.

liberty.c

@@ -2748,16 +2748,12 @@ utf8_decode (const char **s, size_t len)
 	}
 
 	// In the middle of a character
-	if (sequence_len == 1)
+	// or an overlong sequence (subset, possibly MUTF-8, not supported)
+	if (sequence_len == 1 || *p == 0xC0 || *p == 0xC1)
 		return -1;
 
 	// Check the rest of the sequence
 	uint32_t cp = *p++ & ~mask;
-
-	// Overlong sequence (possibly MUTF-8, not supported)
-	if (!cp && sequence_len)
-		return -1;
-
 	while (sequence_len && --sequence_len)
 	{
 		if (p == end)
@@ -4421,7 +4417,7 @@ socket_io_try_write (int socket_fd, struct str *wb)
 //   char    = [\0-\177]  # or any Unicode codepoint in the UTF-8 encoding
 //   escape  = [\\"abfnrtv] / [xX][0-9A-Fa-f][0-9A-Fa-f]? / [0-7][0-7]?[0-7]?
 //
-//   integer = lws '-'? [0-9]+  # whatever strtoll() accepts on your system
+//   integer = lws [-+]? [0-9]+  # whatever strtoll() accepts on your system
 //   null    = lws 'null'
 //   boolean = lws 'yes'  / lws 'YES'  / lws 'no'    / lws 'NO'
 //           / lws 'on'   / lws 'ON'   / lws 'off'   / lws 'OFF'

tests/liberty.c

@@ -331,10 +331,12 @@ test_utf8 (void)
 	soft_assert (utf8_decode (&partial, 1) == -2);
 	soft_assert (utf8_decode (&empty,   0) == -1);
 
-	const char valid[] = "2H₂ + O₂ ⇌ 2H₂O, R = 4.7 kΩ, ⌀ 200 mm";
+	const char valid_1[] = "2H₂ + O₂ ⇌ 2H₂O, R = 4.7 kΩ, ⌀ 200 mm";
+	const char valid_2[] = "\xf0\x93\x82\xb9";
 	const char invalid_1[] = "\xf0\x90\x28\xbc";
 	const char invalid_2[] = "\xc0\x80";
-	soft_assert ( utf8_validate (valid,     sizeof valid));
+	soft_assert ( utf8_validate (valid_1,   sizeof valid_1));
+	soft_assert ( utf8_validate (valid_2,   sizeof valid_2));
 	soft_assert (!utf8_validate (invalid_1, sizeof invalid_1));
 	soft_assert (!utf8_validate (invalid_2, sizeof invalid_2));