lib-managesieve: Don't accept strings with NULs

ManageSieve doesn't allow NULs in strings.

This fixes a bug with unescaping a string with NULs: str_unescape() could
have been called for memory that points outside the allocated string,
causing heap corruption. This could cause crashes or theoretically even
result in remote code execution exploit.

Found by Nick Roessler and Rafi Rubin

src/lib-managesieve/managesieve-parser.c

@@ -258,6 +258,11 @@ managesieve_parser_read_string(struct managesieve_parser *parser,
 			break;
 		}
 
+		if (data[i] == '\0') {
+			parser->error = "NULs not allowed in strings";
+			return FALSE;
+		}
+
 		if (data[i] == '\\') {
 			if (i+1 == data_size) {
 				/* known data ends with '\' - leave it to