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

While working on the previous commit to fix PR cli/28665, I noticed
that the 'edit' command would suffer from the same problem.  That is,
something like:

  (gdb) edit task 123

would cause GDB to break.  For a full explanation of what's going on
here, see the commit message for the previous commit.

As with the previous commit, this issue can be prevented by detecting,
and throwing, a junk at the end of the line error earlier, before
calling decode_line_1.

So, that's what this commit does.  I've also added some tests for this
issue.

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

gdb/cli/cli-cmds.c

@@ -968,6 +968,10 @@ edit_command (const char *arg, int from_tty)
       arg1 = arg;
       event_location_up location = string_to_event_location (&arg1,
 							     current_language);
+
+      if (*arg1)
+	error (_("Junk at end of line specification."));
+
       std::vector<symtab_and_line> sals = decode_line_1 (location.get (),
 							 DECODE_LINE_LIST_MODE,
 							 NULL, NULL, 0);
@@ -987,9 +991,6 @@ edit_command (const char *arg, int from_tty)
 
       sal = sals[0];
 
-      if (*arg1)
-	error (_("Junk at end of line specification."));
-
       /* If line was specified by address, first print exactly which
 	 line, and which file.  In this case, sal.symtab == 0 means
 	 address is outside of all known source files, not that user