match as "" when NULL, to correspond with the Varnish native regexen

tests for failure if the name parameter is NULL or empty

for example if set from an unset header

Varnish regression tests

the match() function

deleted when the RE2 object that owns it is deleted

for sub()

diagnostic when .compile() fails

must be called in vcl_init, all .add()s before .compile(), .compile()
exactly once, and .match() fails if .compile() was not called