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>
diff --git a/common/env_callback.c b/common/env_callback.c
index f4d3dbd..1957cc1 100644
--- a/common/env_callback.c
+++ b/common/env_callback.c
@@ -97,6 +97,7 @@
 
 	e.key	= name;
 	e.data	= NULL;
+	e.callback = NULL;
 	hsearch_r(e, FIND, &ep, &env_htab, 0);
 
 	/* does the env variable actually exist? */