ext/sodium: sodium_pad(): do not copy any bytes if the string is empty

Spotted by San Zhang, thanks!

Backport from PECL libsodium-php 2.0.13

Author: jedisct1

ext/sodium/libsodium.c

@@ -3402,12 +3402,17 @@ PHP_FUNCTION(sodium_pad)
 	st = 1U;
 	i = 0U;
 	k = unpadded_len;
-	for (j = 0U; j <= xpadded_len; j++) {
-		ZSTR_VAL(padded)[j] = unpadded[i];
-		k -= st;
-		st = (size_t) (~(((( (((uint64_t) k) >> 48) | (((uint64_t) k) >> 32) |
-							 (k >> 16) | k) & 0xffff) - 1U) >> 16)) & 1U;
-		i += st;
+	if (unpadded_len > 0) {
+		st = 1U;
+		i = 0U;
+		k = unpadded_len;
+		for (j = 0U; j <= xpadded_len; j++) {
+			ZSTR_VAL(padded)[j] = unpadded[i];
+			k -= st;
+			st = (size_t) (~(((( (((uint64_t) k) >> 48) | (((uint64_t) k) >> 32) |
+								 (k >> 16) | k) & 0xffff) - 1U) >> 16)) & 1U;
+			i += st;
+		}
 	}
 #if SODIUM_LIBRARY_VERSION_MAJOR > 9 || (SODIUM_LIBRARY_VERSION_MAJOR == 9 && SODIUM_LIBRARY_VERSION_MINOR >= 6)
 	if (sodium_pad(NULL, (unsigned char *) ZSTR_VAL(padded), unpadded_len,