Merge pull request #4480 from pan-/fix_time_return

rtc_time: Fix incorrect return value when the calendar is not available.

Author: sg-

platform/mbed_rtc_time.cpp

@@ -53,7 +53,7 @@ time_t time(time_t *timer)
         }
     }
 
-    time_t t = 0;
+    time_t t = -1;
     if (_rtc_read != NULL) {
         t = _rtc_read();
     }