Return NULL from plist_copy() if passed a NULL pointer
Return NULL from plist_copy() if passed a NULL pointer

This will prevent an assert if a NULL pointer is passed, and can make
writing some code easier and cleaner without the need for a NULL check.
For example, plist_copy(plist_dict_get_item(dict, "abc")) would give us
a copy of the dict's node if the dict has a value for the given key, or
NULL without any further checks.

libimobiledevice/libplist,libimobiledevice-win32/libplist,libimobiledevice/libplist,libimobiledevice/libplist,libimobiledevice-win32/libplist,libimobiledevice-win32/libplist,libimobiledevice-win32/libplist