nulstr-util: use memdup_suffix0() where appropriate

if the nulstr is not nul-terminated, we shouldn't use strndup() but
memdup_suffix0(), to not trip up static analyzers which imply we are
duping a string here.

src/basic/nulstr-util.c

@@ -37,7 +37,7 @@ char** strv_parse_nulstr(const char *s, size_t l) {
 
                 e = memchr(p, 0, s + l - p);
 
-                v[i] = strndup(p, e ? e - p : s + l - p);
+                v[i] = memdup_suffix0(p, e ? e - p : s + l - p);
                 if (!v[i])
                         return NULL;