[perl #112316] Make strict vars respect null-to-null assignment

[perl #112316] Make strict vars respect null-to-null assignment

This is a follow-up to commits 6379d4a9a and 862504fb08.

As Karl Williamson (thank you!) pointed out, my changes were not suf-
ficient, because strEQ was still being used, which stops at the first
null, treating "foo\0bar" and "foo\0foo" as equivalent.

Under threads, strict vars was not respecting glob assignment from a
package with a null in its name to another also with a null in its
name, if the two package names shared a common prefix up to the null.