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.