Ensure that caller respects the start argument

	Previously, if there were ignored frames (iseq without pc), we could
	go beyond the requested start frame.  This has two changes:

	1) Ensure that we don't look beyond the start frame by using
	last_cfp = RUBY_VM_PREVIOUS_CONTROL_FRAME(last_cfp) until the
	desired start frame is reached.

	2) To fix the failures caused by change 1), which occur when a
	limited number of frames is requested, scan the VM stack before
	allocating backtrace frames, looking for ignored frames. This
	is complicated if there are ignored frames before and after
	the start, in which case we need to scan until the start frame,
	and then scan backwards, decrementing the start value until we
	get to the point where start will result in the number of
	requested frames.

	This fixes a Rails test failure.  Jean Boussier was able to
	to produce a failing test case outside of Rails.

Co-authored-by: Jean Boussier <jean.boussier@gmail.com>
	---
	 test/ruby/test_backtrace.rb | 16 ++++++++++++++
	 vm_backtrace.c              | 52 +++++++++++++++++++++++++++++++++++++++++++--
	 2 files changed, 66 insertions(+), 2 deletions(-)