Update dlgexpr.c

Regarding Debugger issue.
Modifying memory by double-clicking on a symbol representation of any value in the Memory window (in the right column of symbols) is not working or crashes debugger with a memory access violation in the line 78 of "bld\aui\c\dlgnew.c"
 dlgnew->buff[0] = '\0'; 
because 'buf' in this case points to read only memory of the empty string "" from first argument of DlgGetItemWithRtn( "", ...