Division by zero if a 0-width RAW rect is sent

[jalfd]

In display_raw() in rfb.js, the expression ws.rQlen()/(FBU.width * fb_Bpp) is problematic if FBU.width is zero.

Obviously, the server should not be sending empty rects in the first place, but we came across a problem where it happened, and currently it causes NoVNC to disconnect entirely.

[samhed]

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