[nrf fromtree] net: lib: lwm2m: lwm2m_rw_senml_cbor: only assign time on get_s64() success

Currently GCC complains that temp64 may be used uninitialized in this
 function. Adds a check to ensure time is valid before assignining
 and fixes GCC warning.

Signed-off-by: Brandon Allen <[email protected]>
(cherry picked from commit bb24c83)

Author: brandon-exact

subsys/net/lib/lwm2m/lwm2m_rw_senml_cbor.c

@@ -601,7 +601,9 @@ static int get_time(struct lwm2m_input_context *in, time_t *value)
 	int ret;
 
 	ret = get_s64(in, &temp64);
-	*value = (time_t)temp64;
+	if (ret == 0) {
+		*value = (time_t)temp64;
+	}
 
 	return ret;
 }