EXT-8628 FIXED Crash in LLVOAvatarSelf::getAttachedPointName().

The crash was reproducible only on startup. Apparently, gAgentAvatarp was not valid at that point.
Worked around by checking gAgentAvatarp for being valid.

I didn't investigate what the root cause of the problem was (probably the new multi-attachments implementation), I just needed my viewer to work.

Reviewed by Seraph at https://codereview.productengine.com/secondlife/r/847/

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branch : product-engine