Refactor qemu_xkeymap_mapping_table() to have a single exit point,
so we can easily free the memory allocated by XGetAtomName().

This fixes when running a binary configured with --enable-sanitizers:

  Direct leak of 22 byte(s) in 1 object(s) allocated from:
      #0 0x561344a7473f in malloc (qemu-system-x86_64+0x1dab73f)
      #1 0x7fa4d9dc08aa in XGetAtomName (/lib64/libX11.so.6+0x2a8aa)

Fixes: 2ec78706 ("ui: convert GTK and SDL1 frontends to keycodemapdb")
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20210430155009.259755-1-philmd@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>