kvpair_sort: sanity check to forbid NULL pointer

Technically, we're not allowed to pass NULL to qsort(), even if
nmemb==0:
    Where an argument declared as size_t nmemb specifies the length of
    the array for a function, nmemb can have the value zero...
    Pointer arguments on such a call shall still have valid values, as
    described in 7.1.4.
    C99, 7.20.5/1 Searching and sorting utilities
and:
    If an argument to a function has an invalid value (such as...
    a null pointer...) ... the behavior is undefined.
    C99, 7.1.4/1 Use of library functions

lib/datastruct/kvpair.c

@@ -1,3 +1,4 @@
+#include <assert.h>
 #include <stdlib.h>
 
 #include "kvldskey.h"
@@ -45,6 +46,9 @@ void
 kvpair_sort(struct kvpair * pairs, size_t npairs, size_t mlen)
 {
 
+	/* Sanity check. */
+	assert(pairs != NULL);
+
 	/* See above comments about cookie_mlen and non-threadsafety. */
 	cookie_mlen = mlen;