fix(xslt): fix issue 746 (transform() crashes Code Coverage)

[AirQuick]

Fixes #746

These are outside the scope of the fix:

• Coverage report HTML when stylesheet-location points to another XSLT file
• xsl:for-each/@saxon:threads

[galtm]

@AirQuick : I plan to review this by next Monday. I'll have to dig up the original situation that led me to #746, but I suspect that fixing only the case where stylesheet-location points to the same XSLT file is still a useful step forward.

[AirQuick]

Thanks, @galtm.
The fix will work even when stylesheet-location points to another stylesheet. However, the coverage reporter (src/reporter/coverage-report.xsl) will not able to find it.
coverage-report.xsl collects stylesheets only through xsl:include or xsl:import:

xspec/src/reporter/coverage-report.xsl

Lines 353 to 356 in 5cbcac4

	<xsl:variable name="imports" as="document-node()*"
	select="document($stylesheets/*/(xsl:import|xsl:include)/@href)" />
	<xsl:variable name="new-stylesheets" as="document-node()*"
	select="$stylesheets | $imports" />

[galtm]

I see what you mean. For my purpose, not including the coverage of the other stylesheet in the HTML report is fine. (I can see data about the other stylesheet in the trace file (...-coverage.xml), so there could be a path toward including the other stylesheet's coverage in the HTML report if someone wanted that.)

[galtm]

Looks good. Thank you for fixing this!