common: env: initialize scalar variable

Before calling hsearch_r, initialize callback entry to NULL.

Coverity log:
"
Uninitialized scalar variable (UNINIT)
uninit_use_in_call: Using uninitialized value e.
Field e.callback is uninitialized when calling hsearch_r.
"

Reported-by: Coverity
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Cc: Tom Rini <trini@konsulko.com>
Cc: Simon Glass <sjg@chromium.org>