validate utf-8 string

cc -DHAVE_CONFIG_H -I. -I..  -I.. -I../tests    -O2 -pipe  -fstack-protector-strong -fno-strict-aliasing  -Wall -Werror -Wcast-qual -Wno-error=deprecated-declarations -Wextra -Wno-unused-parameter -D_GNU_SOURCE -D_REENTRANT -MT test_parse.o -MD -MP -MF .deps/test_parse.Tpo -c -o test_parse.o test_parse.c
test_parse.c:256:14: error: adding 'int' to a string does not append to the string [-Werror,-Wstring-plus-int]
        { "null123" + 4,       4, 3, json_tokener_success, 1 },
          ~~~~~~~~~~^~~
test_parse.c:256:14: note: use array indexing to silence this warning
        { "null123" + 4,       4, 3, json_tokener_success, 1 },
                    ^
          &         [  ]
test_parse.c:258:12: error: adding 'int' to a string does not append to the string [-Werror,-Wstring-plus-int]
        { "nullx" + 4,         2, 0, json_tokener_error_parse_unexpected, 1 },
          ~~~~~~~~^~~
test_parse.c:258:12: note: use array indexing to silence this warning
        { "nullx" + 4,         2, 0, json_tokener_error_parse_unexpected, 1 },
                  ^
          &       [  ]
test_parse.c:260:25: error: adding 'int' to a string does not append to the string [-Werror,-Wstring-plus-int]
        { "{\"a\":1}{\"b\":2}" + 7,
          ~~~~~~~~~~~~~~~~~~~~~^~~
test_parse.c:260:25: note: use array indexing to silence this warning
        { "{\"a\":1}{\"b\":2}" + 7,
                               ^
          &                    [  ]
3 errors generated.
*** Error code 1

Stop.

Increased the test coverage of printbuf.c 82% to 92%.

update pointer test case

update tsetcase for tokener_c

update json_visit testcase

Contrary to other CMAKE variables the CMAKE_C_FLAGS variable is the
composed string of flags for the C compiler. It is therefore not a list
to append to. Current implementation results in these incorrect CFLAGS,
e.g., "-O2 -g -fblahblah;-UNDEBUG". Extending the CFLAGS this way
results in the proper CFLAGS, e.g., "-O2 -g -fblahblah -UNDEBUG".

undefine NDEBUG for tests

(closes #406)

test/Makefile.am:add missing deps for test1 and test2

Allows the tests to pass when running `make distcheck`

This fixes the 2 broken tests I mentioned at
https://github.com/json-c/json-c/pull/499#discussion_r306998261

Also try to accomodate formats to explicitly exclude the decimal (e.g. %.0f).