libs/libc: posix_spawnattr_init zero-initialize attr

Zero-initialize posix_spawnattr_t in posix_spawnattr_init().
Prevent use of uninitialized fields reported by Coverity.
Preserve existing default flag initialization.

Signed-off-by: fangxinyong <fangxinyong@xiaomi.com>

libs/libc/spawn/lib_psa_init.c

@@ -61,6 +61,8 @@ int posix_spawnattr_init(posix_spawnattr_t *attr)
 
   DEBUGASSERT(attr);
 
+  memset(attr, 0, sizeof(*attr));
+
   /* Flags: None */
 
   attr->flags = 0;