avro

[AVRO-3945] Add missing bounds check in the loop (#2812)

* [AVRO-3945] Add missing bounds checks for extra increments in the loop

This issue was found by cppcheck:

    impl/json/JsonIO.cc:319:66: warning: Missing bounds check for extra iterator increment in loop. [StlMissingComparison]
        for (string::const_iterator it = s.begin(); it != s.end(); ++it) {
                                                                     ^
    impl/json/JsonIO.cc:350:37: note: Missing bounds check for extra iterator increment in loop.
                            char c = *++it;
                                        ^
    impl/json/JsonIO.cc:319:66: note: Missing bounds check for extra iterator increment in loop.
        for (string::const_iterator it = s.begin(); it != s.end(); ++it) {

The original implementation contained a for-loop that incremented an
iterator on each iteration **and** if a backslash was found. This caused
a situtation when a malicious string could cause an invalid memory access,
because the iterator would reach **after** the `s.cend()` due to additional
increments in the loop body.

This commit fixes the issue.

* build.sh: sort unittests and add forgotten tests