common: cli: avoid memory leak

Whether CONFIG_SYS_HUSH_PARSER is defined or not, should always
check to free 'buff' to avoid memory leak.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Cc: Tom Rini <trini@konsulko.com>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Cc: Simon Glass <sjg@chromium.org>