net: lwm2m: fix out-of-bounds access in put_s8()

Per Coverity report, oma_tlv_put() does pointer arithmetic accessing
the data as an array of u8_t.  In put_bool() we get a singleton
pointer from the evaluation of: "value != 0 ? 1 : 0" which is
passed to put_s8() which in turn passes it to oma_tlv_put().

To avoid misinterpretation, let's create a temporary s8_t variable
to pass into oma_tlv_put instead.

Fixes: #12312

Signed-off-by: Michael Scott <[email protected]>

Author: mike-scott

subsys/net/lib/lwm2m/lwm2m_rw_oma_tlv.c

@@ -554,7 +554,9 @@ static size_t put_float64fix(struct lwm2m_output_context *out,
 static size_t put_bool(struct lwm2m_output_context *out,
 		       struct lwm2m_obj_path *path, bool value)
 {
-	return put_s8(out, path, value != 0 ? 1 : 0);
+	s8_t value_s8 = (value != 0 ? 1 : 0);
+
+	return put_s8(out, path, value_s8);
 }
 
 static size_t get_number(struct lwm2m_input_context *in, s64_t *value,