dtrace: Fix /"string" == NULL/ comparisons using an uninitialized value.

A test of this is funcs/tst.strtok.d which has this filter:

BEGIN
/(this->field = strtok(this->str, ",")) == NULL/
{
        exit(1);
}

The test will randomly fail with exit status of 1 indicating that this->field
was NULL even though printing it out shows it is not.

This is compiled to the DTrace instruction set:

// Pushed arguments not shown here
// call strtok() and set result into %r1
07: 2f001f01    call DIF_SUBR(31), %r1          ! strtok
// set thread local scalar this->field from %r1
08: 39050101    stls %r1, DT_VAR(1281)          ! DT_VAR(1281) = "field"
// Prepare for the == comparison
// Set right side of %r2 to NULL
09: 25000102    setx DT_INTEGER[1], %r2         ! 0x0
// string compare %r1 (strtok result) to %r2
10: 27010200    scmp %r1, %r2

In this case only %r1 is loaded with a string limit set to lim1. %r2 being
NULL does not get loaded and does not set lim2. Then we call dtrace_strncmp()
with MIN(lim1, lim2) resulting in passing 0 and comparing neither side.
dtrace_strncmp() handles this case fine and it already has been while
being lucky with what lim2 was [un]initialized as.

Reviewed by: markj, Don Morris <dgmorris AT earthlink.net>
Sponsored by: Dell EMC
Differential Revision: https://reviews.freebsd.org/D27671