uci: libuci leaking memory on non-existent config file

In file file.c function uci_file_load() allocates memory for filename.
When uci_open_stream() failed with exception this memory was never freed.
This patch makes call to uci_open_stream() wrapped in exception handling
inside uci_file_load() where filename is freed.

Signed-off-by: Krzysztof Kuznik <k.kuznik@avsystem.com>