Factorise the code to parse conditions of statements, i.e. an expression enclosed...

Factorise the code to parse conditions of statements, i.e. an expression enclosed in parentheses.

This addresses several issues:
- No null pointer as condition anymore, if a if lacks its (.
- No anchor leak anymore, if a switch lacks its ).
- Better error recovery: No error statement anymore, if any part of a condition is missing.
- Less code duplication.