When executing a Line Number program statement, the state machine bails
out a few bytes before the current offset reaches the end of section to
account for possible padding bytes at the end.

The current test is checking if current_offset + 4 >= section_length,
which is too early for e.g. a program terminating with

  Advance PC by constant 17 to 0x776
  Extended opcode 1: End of Sequence

at offset 101 for a section with length 105.

This change tightens the computation, allowing proper interpretation
in the example case above, and explains the rationale.

Tested on x86_64-pc-linux-gnu, committed on trunk

2017-12-05  Olivier Hainque  <hainque@adacore.com>

	* s-dwalin.adb (Read_And_Execute_Isn): Adjust test checking for the end
	of section. Add comments explaining the rationale of the computation.