Omit null-pointer test in intervals.h FRAME

Omit null-pointer test in intervals.h FRAME

* src/intervals.h (ROOT_INTERVAL_P, ONLY_INTERVAL_P)
(INTERVAL_LAST_POS): Omit unnecessary parens.
(LENGTH): Omit test for null pointer.  The argument is never null.
The unnecessary test causes GCC 7.1.0 to assume that the argument
might be null, and therefore to issue false alarms when the
argument is dereferenced in other expressions.