Fix undefined behavior in timestamp parsing

If this algorithm ever processed a string where the + came before the .,
then dot - plus would render a negative, and overflow, causing a crash.

In practice, this doesn't happen.

Tested: Inspection only at this time.  Difficult to trigger error.

Signed-off-by: Ed Tanous <edtanous@google.com>
Change-Id: Ic7f5153d144ac551118c4f4b2d61f82626ac3779
diff --git a/redfish-core/include/event_service_manager.hpp b/redfish-core/include/event_service_manager.hpp
index 1785828..7e249ba 100644
--- a/redfish-core/include/event_service_manager.hpp
+++ b/redfish-core/include/event_service_manager.hpp
@@ -247,7 +247,7 @@
     // RFC3339 format which matches the Redfish format except for the
     // fractional seconds between the '.' and the '+', so just remove them.
     std::size_t dot = timestamp.find_first_of('.');
-    std::size_t plus = timestamp.find_first_of('+');
+    std::size_t plus = timestamp.find_first_of('+', dot);
     if (dot != std::string::npos && plus != std::string::npos)
     {
         timestamp.erase(dot, plus - dot);