tnef: Fix major issue iterating over attr streams

The original logic would read the count of attributes in the stream and then
read the first attribute in the constructor of TnefAttributeStreamReader.
Then the iterator interface would call ::rewind() before iterating to the
first item. rewind() set the @pos attribute to zero, which would cause the
attribute count (4-byte int) to be interpreted incorrectly as part of the
first attribute.

The new logic sets the position at 4 after rewind()ing, and does not read
the first attribute twice. It also properly detects the end of the attribute
stream by the number of attributes advertised as the first four bytes of the
stream (read into the @count attribute initially).
Merge pull request #825 from greezybacon/issue/tnef-bad-rewind

tnef: Fix major issue iterating over attar streams

Reviewed-By: Peter Rotich <peter@osticket.com>