rate_control.cpp:removed redundant NULL checks

First argument (VideoEncData *) of the functions RC_UpdateBuffer, targetBitCalculation and
calculateQuantizer_Multipass in rate_control.cpp file of libstagefright_m4vh263enc SoftMPEG4Encoder
was checked against NULL after it was already refered successfully.
Hence, those redundant checks are removed now.  Above mentioned functions' VideoEncData pointer
argument can't be null as those functions are called only after successful initialisation in
PVInitVideoEncoder by SoftMPEG4Encoder::initEncoder.
rate_control.cpp is compiled as a part of static library : libstagefright_m4vh263enc.
This library can only be used using SoftMPEG4Encoder wrapper which makes sure VideoEncData * was
allocated required memory.

Bug: 112154365

Test: Encoded a YUV file using test case libstagefright_m4vh263enc_test and generated a bit stream
      output file without any error.

Change-Id: Id7f08cc57350bc70729a82afebbc79ba83dac72d