selinux: remove redundant assignment to str

str is being assigned to an empty string but str is never being
read after that, so the assignment is redundant and can be removed.
Moving the declaration of str to a more localised block, cleans up
clang warning: "Value stored to 'str' is never read"
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>

security/selinux/hooks.c

@@ -3176,18 +3176,17 @@ static int selinux_inode_setxattr(struct dentry *dentry, const char *name,
 		if (!has_cap_mac_admin(true)) {
 			struct audit_buffer *ab;
 			size_t audit_size;
-			const char *str;
 
 			/* We strip a nul only if it is at the end, otherwise the
 			 * context contains a nul and we should audit that */
 			if (value) {
-				str = value;
+				const char *str = value;
+
 				if (str[size - 1] == '\0')
 					audit_size = size - 1;
 				else
 					audit_size = size;
 			} else {
-				str = "";
 				audit_size = 0;
 			}
 			ab = audit_log_start(current->audit_context, GFP_ATOMIC, AUDIT_SELINUX_ERR);