gdb/testsuite: initialize "correct" variable in gdb.cp/cpexprs.exp.tcl

gdb/testsuite: initialize "correct" variable in gdb.cp/cpexprs.exp.tcl

Due to a GDB bug (visible when building with -D_GLIBCXX_DEBUG), GDB
crashes somewhere in the middle of gdb.cp/cpexprs.exp, and thus fails to
read the string, at gdb.cp/cpexprs.exp.tcl:725.  The "correct" variable
doesn't get set, and I then see this TCL error:

  ERROR: can't read "correct": no such variable

Avoid the TCL error by initializing the "correct" variable to a dummy
value.

Change-Id: I828968d9b2d105ef47f8da2ef598aa16a518c059