pairmake w/ regexps in master

[Phil Mayers]

I think "pairmake" might not be quite right in master:

https://github.com/FreeRADIUS/freeradius-server/blob/master/src/lib/valuepair.c#L1640

If "vp->operator" is T_OP_REG_{EQ,NE} then:

  1. value is checked - if null, error
  2. pairmake_xlat is called
  3. this calls pairmake with value==NULL - goto 1, fail

This causes problems with regexps in users files e.g.

DEFAULT	Attr =~ "some(.+)"

...which error out at startup with:

Parse error (check) for entry blah: No regular expression found in ICzone