flush buffer in serializer::dump_escaped case UTF8_REJECT

serializer use fixed buffer. Whenever it is nearly full, it is flushed to `output_adapter_t<char> o`
But the code forgets to flush when there is a invalid utf8 code point
So there will be buffer overflow.

test/Makefile

@@ -30,6 +30,7 @@ SOURCES = src/unit.cpp \
           src/unit-items.cpp \
           src/unit-iterators1.cpp \
           src/unit-iterators2.cpp \
+          src/unit-invalid-utf8.cpp \
           src/unit-merge_patch.cpp \
           src/unit-json_patch.cpp \
           src/unit-json_pointer.cpp \