[Patch] Remove some redundant NULL pointer checks.

[Paolo Carlini]

Hi,

the below, blessed by Nathan (see his original message
concerning in_avail()), removes many redundant NULL pointer
checks from iostream functions.

Basically, the point is that if any of _M_out_* (_M_in_*)
is NULL all the others are NULL too.

I took the occasion, to also const-qualify
some boolean variables.

Tested x86-linux.

Benjamin, do you like it for trunk?

Thanks,
Paolo.

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