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>

common/env_callback.c

@@ -97,6 +97,7 @@ static int set_callback(const char *name, const char *value, void *priv)
 
 	e.key	= name;
 	e.data	= NULL;
+	e.callback = NULL;
 	hsearch_r(e, FIND, &ep, &env_htab, 0);
 
 	/* does the env variable actually exist? */

common/env_flags.c

@@ -455,6 +455,7 @@ static int set_flags(const char *name, const char *value, void *priv)
 
 	e.key	= name;
 	e.data	= NULL;
+	e.callback = NULL;
 	hsearch_r(e, FIND, &ep, &env_htab, 0);
 
 	/* does the env variable actually exist? */