Fixed: now skipping correct number of values in magnitude buffer

Variable j points to the current location in the magnitude vector.
When decoding a message  (MODES_PREAMBLE_US+msglen)*2 is added to j.
In the loop head j is increased by 1, so one value was skipped.
Merge pull request #14 from kopppa91/master

Fixed: now skipping correct number of values in magnitude buffer