Fix regression in Ada aggregate assignment

A recent upstream patch of mine caused a regression in aggregate
assignment.  The bug was that add_component_interval didn't properly
update the array contents in one resize case.

I found furthermore that there was no test case that would provoke
this failure.  This patch fixes the bug and introduces a test.

gdb/ChangeLog
2021-01-07  Tom Tromey  <tromey@adacore.com>

	* ada-lang.c (add_component_interval): Start loop using vector's
	updated size.

gdb/testsuite/ChangeLog
2021-01-07  Tom Tromey  <tromey@adacore.com>

	* gdb.ada/assign_arr.exp: Add 'others' test.