dtm.cpp: Add more checks during load()
dtm.cpp: Add more checks during load()

* Refuse to load a file with zero patterns.
* Fail if an instrument name is too long.
* Ignore instrument events with invalid instrument number.
* Fail if order list contains invalid pattern numbers or
  restart position.

This commit fixes invalid memory accesses for the following
fuzzing samples:
  i-110_060.dtm, i-110_128.dtm.

Bug: https://github.com/adplug/adplug/issues/110

adplug/adplug,adplug/adplug