[gdb] Don't return non-existing path in debuginfod_source_query

[gdb] Don't return non-existing path in debuginfod_source_query

When setting env var DEBUGINFOD_URLS to " " and running the testsuite, we run
into these regressions:
...
FAIL: gdb.base/list-missing-source.exp: info source
FAIL: gdb.base/source-dir.exp: info source before setting directory search list
...

Setting var DEBUGINFOD_URLS to " " allows the debuginfod query function
debuginfod_source_query to get past its early exit.

The function debuginfod_source_query is documented as: "If the file is
successfully retrieved, its path on the local machine is stored in DESTNAME".

However, in case we get back -ENOENT from libdebuginfod, we still set
DESTNAME:
....
  if (fd.get () < 0 && fd.get () != -ENOENT)
    printf_filtered (_("Download failed: %s.  Continuing without source file %ps.\n"),
                     safe_strerror (-fd.get ()),
                     styled_string (file_name_style.style (),  srcpath));
  else
    *destname = make_unique_xstrdup (srcpath);

  return fd;
...

Fix this by making debuginfod_source_query fit it's documentation and only
setting DESTNAME when successfully retrieving a file.  Likewise in
debuginfod_debuginfo_query.

gdb/ChangeLog:

2020-11-23  Tom de Vries  <tdevries@suse.de>

	* debuginfod-support.c (debuginfod_source_query)
	(debuginfod_debuginfo_query): Only set DESTNAME if successful.