This fixes multiple bugs found in the partial backtrace
optimization added in 3b24b791.
These bugs occurs when passing a start argument to caller where
the start argument lands on a iseq frame without a pc.

Before this commit, the following code results in the same
line being printed twice, both for the #each method.

```
def a; [1].group_by { b } end
def b; puts(caller(2, 1).first, caller(3, 1).first) end
a
```

After this commit and in Ruby 2.7, the lines are different,
with the first line being for each and the second for group_by.

Before this commit, the following code can either segfault or
result in an infinite loop:

```
def foo
  caller_locations(2, 1).inspect # segfault
  caller_locations(2, 1)[0].path # infinite loop
end

1.times.map { 1.times.map { foo } }
```

After this commit, this code works correctly.

In terms of the implementation, this correctly skips iseq frames
without pc that occur before the number of frames the caller
requested to skip.

This rewrites the algorithm used for handling the partial
backtraces.  It scans from the current frame outward to the
earliest frame, until it has found the desired number of frames.
It records that frame as the start frame.  If needed, it continues
scanning backwards until arg->prev_cfp is set, as that is needed
to set the location of the first frame. Due to the fact that arg
is a void pointer, it's not possible to check this directly, but
this calls the iter_skip function in a situation where it knows
it will set arg->prev_cfp, and then breaks out of the loop.

Fixes [Bug #18053]