qemu-config: fix memory leak on ferror()

qemu-config: fix memory leak on ferror()

The leak is basically impossible to reach, since the only common way
to get ferror(fp) is by passing a directory to -readconfig.  In that
case, the error occurs before qdict is set to anything non-NULL.
However, it's theoretically possible to get there after an EIO.

Cc: [email protected]
Reported-by: Peter Maydell <[email protected]>
Fixes: f7544edcd3 ("qemu-config: add error propagation to qemu_config_parse", 2021-03-06)
Signed-off-by: Paolo Bonzini <[email protected]>