[pargue]

Fail json_dumps when an object key cannot be written

Fixes #655.

do_dump ignores dump_string's return value for object keys, on both the JSON_SORT_KEYS and the unsorted path. When the output buffer cannot grow for a long key, the key is dropped, but the ": " and the value after it are small enough to fit, so json_dumps returns malformed JSON such as {": 1, "a": 2} instead of NULL.

This checks dump_string's result at both sites, as is already done for string values.

Behaviour change: two cases now make json_dumps / json_dump_callback / json_dumpf fail instead of producing malformed output:

• a key that cannot be written: out of memory, or a dump callback or file write that returns an error. The callback docs already say returning -1 stops the encoding; for keys it did not.
• a key that is not valid UTF-8. Only the _nocheck setters can create one. On master {"ab\xff": 1, "x": 2} dumps as {": 1, "x": 2}; it now fails, the same as an invalid string value.

json_dumpb is unaffected (its callback never fails). The sorted path still frees its key array on the new early return.

Tests in test/suites/api/test_dump.c: dump_key_out_of_memory dumps an object with a 4 KB key through an allocator that refuses blocks over 1 KB, and dump_key_invalid_utf8 dumps one with an invalid UTF-8 key, each sorted and unsorted. Both fail on master and pass with the fix. The full suite passes (215 tests), also under ASan/UBSan.