SPARK-4222 [CORE] use readFully in FixedLengthBinaryRecordReader

replaces the existing read() call with readFully().

Author: industrial-sloth <industrial-sloth@users.noreply.github.com>

Closes #3093 from industrial-sloth/branch-1.2-fixedLenRecRdr and squashes the following commits:

a245c8a [industrial-sloth] use readFully in FixedLengthBinaryRecordReader

(cherry picked from commit 6844e7a8219ac78790a422ffd5054924e7d2bea1)
Signed-off-by: Matei Zaharia <matei@databricks.com>