Use common_val_print_checked in print_variable_and_value

Use common_val_print_checked in print_variable_and_value

GDB was crashing in "bt full" for a large Ada program.  I tracked this
down to a combination of bugs.

The first bug, fixed here, is that print_variable_and_value calls
common_val_print.  Normally only the value-printing implementation
should call this; from the top level, common_val_print_checked is
preferred, because it correctly handles values that are wholly
optimized out.

I wasn't able to write a test case for this.

gdb/ChangeLog
2021-04-15  Tom Tromey  <tromey@adacore.com>

	* printcmd.c (print_variable_and_value): Use
	common_val_print_checked.