Avoid crash in Ada value printing with optimized-out array

Avoid crash in Ada value printing with optimized-out array

The Ada value-printing code could crash when printing an array which
had been optimized out.

The crash is difficult to reproduce, but I did manage to write a test
that at least shows that the previous behavior was incorrect -- before
the patch, the array is printed as if it is valid and every value is 0.

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

	* ada-valprint.c (ada_value_print_array): Handle optimized-out
	arrays.

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

	* gdb.dwarf2/arr-stride.exp: Add test.