#86 NullPointerException with -T tracing

[Michael Kay]

When Saxon is run with the tracing (-T) option, a
NullPointerException occurs if <xsl:for-each> is used in
the definition of a global variable.

Occurs in 7.1, 6.5.2, and probably in all earlier releases.

Source code fixed in both branches.

Test case added (bug90), though it needs to be run
manually with -T option to demonstrate the problem.

[Michael Kay]

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Fixed in 7.2, still present in 6.5.2