StdLib: Fix GCC warnings/errors caused by variables being set but not used.

Removed variables that had no effect on code behavior.

Fifo.c::FIFO_Dequeue: Replaced instances of "Self->ElementSize" with preexisting variable "SizeOfElement".

IIOutilities.c::IIO_GetInChar: Fixed variable of wrong, but compatible, type and made updating of housekeeping variables dependent upon successful completion of reading from the buffer.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Olivier Martin <olivier.martin@arm.com>
Reviewed by: Daryl McDaniel <daryl.mcdaniel@intel.com>

git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@16276 6f19259b-4bc3-4df7-8a09-765794883524