file: uci_file_commit: fix memory leak

file: uci_file_commit: fix memory leak

Fixes following memory leak:

 26 bytes in 1 blocks are definitely lost in loss record 1 of 1
   at 0x4C2FB0F: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
   by 0x52DA68F: vasprintf (vasprintf.c:73)
   by 0x52B71D3: asprintf (asprintf.c:35)
   by 0x4E40F67: uci_file_commit (file.c:738)
   by 0x4E3FD94: uci_commit (libuci.c:193)
   by 0x401ED9: uci_do_import (cli.c:408)
   by 0x401ED9: uci_cmd (cli.c:685)
   by 0x4016FA: main (cli.c:776)

Signed-off-by: Petr Štetiar <ynezz@true.cz>