[PR modula2/120731] error in Strings.Pos causing sigsegv

[PR modula2/120731] error in Strings.Pos causing sigsegv

This patch corrects the m2log library procedure function
Strings.Pos which incorrectly sliced the wrong component
of the source string.  The incorrect slice could cause
a sigsegv if negative slice indices were generated.

gcc/m2/ChangeLog:

	PR modula2/120731
	* gm2-libs-log/Strings.def (Delete): Rewrite comment.
	* gm2-libs-log/Strings.mod (Pos): Rewrite.
	(PosLower): New procedure function.

gcc/testsuite/ChangeLog:

	PR modula2/120731
	* gm2/pimlib/logitech/run/pass/teststrings.mod: New test.

Signed-off-by: Gaius Mulley <gaiusmod2@gmail.com>