qga: fix compiler warnings (clang 5)

static code analyzer complain:

qga/commands-posix.c:2127:9: warning: Null pointer passed as an argument to a 'nonnull' parameter
        closedir(dp);
        ^~~~~~~~~~~~

Reported-by: Clang Static Analyzer
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>