gdb: handle calls to list command passing only a linespec condition

In PR cli/28665, it was reported that GDB would crash when given a
command like:

  (gdb) list task 123

The problem here is that in cli/cli-cmd.c:list_command, the string
'task 123' is passed to string_to_event_location in find a location
specification.  However, this location parsing understands about
breakpoint conditions, and so, will stop parsing when it sees
something that looks like a condition, in this case, the 'task 123'
looks like a breakpoint condition.

As a result, the location we get back from string_to_event_location
has no actual location specification attached to it.  The actual call
path is:

  list_command
    string_to_event_location
      string_to_event_location_basic
        new_linespec_location

In new_linespec_location we call linespec_lex_to_end, which looks at
'task 123' and decides that there's nothing there that describes a
location.  As such, in new_linespec_location, the spec_string field of
the location is left as nullptr.

Back in list_command we then call decode_line_1, which calls
event_location_to_sals, which calls parse_linespec, which takes the
spec_string we found earlier, and tries to converts this into a list
of sals.

However, parse_linespec is not intended to be passed a nullptr, for
example, calling is_ada_operator will try to access through the
nullptr, causing undefined behaviour.  But there are other cases
within parse_linespec which don't expect to see a nullptr.

When looking at how to fix this issue, I first considered having
linespec_lex_to_end detect the problem.  That function understands
when the first thing in the linespec is a condition keyword, and so,
could throw an error saying something like: "no linespec before
condition keyword", however, this is not going to work, at least, not
without additional changes to GDB, it is valid to place a breakpoint
like:

  (gdb) break task 123

This will place a breakpoint at the current location with the
condition 'task 123', and changing linespec_lex_to_end breaks this
behaviour.

So, next, I considered what would happen if I added a condition to an
otherwise valid list command, this is what I see:

  (gdb) list file.c:1 task 123
  Junk at end of line specification.
  (gdb)

So, then I wondered, could we just pull the "Junk" detection forward,
so that we throw the error earlier, before we call decode_line_1?

It turns out that yes we can.  Well, sort of.

It is simpler, I think, to add a separate check into the list_command
function, after calling string_to_event_location, but before calling
decode_line_1.  We know when we call string_to_event_location that the
string in question is not empty, so, after calling
string_to_event_location, if non of the string has been consumed, then
the content of the string must be junk - it clearly doesn't look like
a location specification.

I've reused the same "Junk at end of line specification." error for
consistency, and added a few tests to cover this issue.

While the first version of this patch was on the mailing list, a
second bug PR gdb/28797 was raised.  This was for a very similar
issue, but this time the problem command was:

  (gdb) list ,,

Here the list command understands about the first comma, list can have
two arguments separated by a comma, and the first argument can be
missing.  So we end up trying to parse the second command "," as a
linespec.

However, in linespec_lex_to_end, we will stop parsing a linespec at a
comma, so, in the above case we end up with an empty linespec (between
the two commas), and, like above, this results in the spec_string
being nullptr.

As with the previous case, I've resolved this issue by adding an extra
check for junk at the end of the line - after parsing (or failing to
parse) the nothing between the two commas, we still have the "," left
at the end of the list command line - when we see this we can throw
the same "junk at the end of the line" error, and all is good.

I've added tests for this case too.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=28665
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=28797

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