[khromenokroman]

@olofhagsand Hello

Description:
Two independent small fixes.

1. XPath mod crashed with SIGFPE

xp_numop() computed mod as ((int)n1)%((int)n2). If the divisor
truncates to 0, e.g. 1 mod 0 or 1 mod 0.5, the process is killed by
SIGFPE.

Such expressions can come from YANG must/when statements evaluated on
user data, or from an XPath filter in a NETCONF <get>, so a client can
crash the backend. Example:

  leaf r {
    type int32;
    must "(../a mod ../b) = 0";
  } 

  <c><a>6</a><b>0</b><r>1</r></c>   -> validate: backend killed by SIGFPE

XPath 1.0 section 3.5 defines mod on numbers (floating point) as the
remainder from a truncating division, i.e. fmod(). Using fmod():

• returns NaN for a zero divisor (like div), so the must above simply
  fails validation;
• gives correct results for non-integer operands: 5.5 mod 2 = 1.5
  (previously 1).

expression	master	this PR
5 mod 2, 5 mod -2, -5 mod 2	1, 1, -1	1, 1, -1
5.5 mod 2	1	1.5
1 mod 0	SIGFPE	NaN
1 mod 0.5	SIGFPE	0
must "(../a mod ../b) = 0", b=0	SIGFPE	Failed MUST xpath

2. xml_diff() with NULL first tree

If x0 == NULL, the whole second tree is new, but x0 (NULL) was appended
to the second vector instead of x1 (copy-paste from the x1 == NULL
case just above). Callers iterating second would get a NULL node.

Testing

• test/test_xpath.sh: added mod cases including zero and non-integer
  divisors (values from XPath 1.0 3.5 examples).
• Verified the expressions above with clixon_util_xpath and a standalone
  validation program against master and this branch.
• Merges cleanly with Fix leafref binary search accepting entries excluded by xpath predicates #700, Fix xp_ctx heap overflow: struct layout depended on XPATH_CURRENT_OPTIMIZE #701 and Fix empty leafref value passing validation without referred instance #702.