Improve test coverage.

* Test equality of different length strings.
* Add tab to json_pack whitespace test.
* Test json_sprintf with empty result and invalid UTF.
* Test json_get_alloc_funcs with NULL arguments.
* Test invalid arguments.
* Add test_chaos to test allocation failure code paths.
* Remove redundant json_is_string checks from json_string_equal and
  json_string_copy.  Both functions are static and can only be called
  with a json string.

Fixes to issues found by test_chaos:
* Fix crash on OOM in pack_unpack.c:read_string().
* Unconditionally free string in string_create upon allocation failure.
  Update load.c:parse_value() to reflect this.  This resolves a leak on
  allocation failure for pack_unpack.c:pack_string() and
  value.c:json_sprintf().

Although not visible from CodeCoverage these changes significantly
increase branch coverage.  Especially in src/value.c where we previously
covered 67.4% of branches and now cover 96.3% of branches.

test/suites/api/test_memory_funcs.c

@@ -122,9 +122,16 @@ static void test_secure_funcs(void)
     create_and_free_complex_object();
 }
 
+static void test_bad_args(void)
+{
+    /* The result of this test is not crashing. */
+    json_get_alloc_funcs(NULL, NULL);
+}
+
 static void run_tests()
 {
     test_simple();
     test_secure_funcs();
     test_oom();
+    test_bad_args();
 }