Fix JNI error in Linux.(pipe2|sendfile).

JNI code in libcore.io.Linux.pipe2 and sendfile
were calling JNIEnv methods even if there was a pending
exception created by throwIfMinusOne helper method.
Fixed the code to return quickly if there's a pending errno
exception.

Test: libcore.io.OsTest
Bug: 65051835
Change-Id: Id921da9e1592b10889c495edb6b4c9919d181e3f
2 files changed