STORM-452 FIXED Made protective fix for crash in LLAgentCamera::resetView().

I managed to reprodcuce the crash once and couldn't reproduce it any time later, so this fix is made judging from line that I found (viewer was in debug mode when it happened)
in call stack after crash. Added check for gMenuHolder against NULL to prevent further crashing. There is no 100% guarantee that this fixes the crash from ticket, but it won't do any harm either.

indra/newview/llagentcamera.cpp

@@ -296,8 +296,11 @@ void LLAgentCamera::resetView(BOOL reset_camera, BOOL change_camera)
 			LLSelectMgr::getInstance()->deselectAll();
 		}
 
-		// Hide all popup menus
-		gMenuHolder->hideMenus();
+		if (gMenuHolder != NULL)
+		{
+			// Hide all popup menus
+			gMenuHolder->hideMenus();
+		}
 	}
 
 	if (change_camera && !gSavedSettings.getBOOL("FreezeTime"))