[SPARK-4107] Fix incorrect handling of read() and skip() return values (branch-1.1 backport)

`read()` may return fewer bytes than requested; when this occurred, the old code would silently return less data than requested, which might cause stream corruption errors.  `skip()` faces similar issues, too.

This patch fixes several cases where we mis-handle these methods' return values.

This is a backport of #2969 to `branch-1.1`.