The server was not cleaning the last IO error and error number when
resetting slave.

This patch addresses this issue by backporting into 5.1 part of the
patch in BUG 34654. A fix for this issue had already been pushed into
6.0 as part of the aforementioned bug, however the patch also included
some refactoring. The fix for 5.1 does not take into account the
refactoring part.