parser: Ensure to preserve CheckArray flag properly

Whenever adjusting expression parser flags, the original state must be
restored afterwards, to not break the overall parser state.

27c1b790 introduced some bad fbSetCheckArray() calls in array index
parsing, which from then on trashed the parser state. The same problem
existed in typeof/sizeof expression parsing.

http://www.freebasic.net/forum/viewtopic.php?f=3&t=21367