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 6844e7a8)
Signed-off-by: Matei Zaharia <matei@databricks.com>

core/src/main/scala/org/apache/spark/input/FixedLengthBinaryRecordReader.scala

@@ -115,7 +115,7 @@ private[spark] class FixedLengthBinaryRecordReader
     if (currentPosition < splitEnd) {
       // setup a buffer to store the record
       val buffer = recordValue.getBytes
-      fileInputStream.read(buffer, 0, recordLength)
+      fileInputStream.readFully(buffer)
       // update our current position
       currentPosition = currentPosition + recordLength
       // return true