Inside strings, All UTF-8 characters except for \, " and Unicode
control codes are dumped as-is. The control codes that have a special
one-character escape use that escape, and other control codes are
dumped using the \uXXXX escape.
Nothing was appended to strbuffer, so the buffer was left empty. An
empty strbuffer is not an empty string but NULL, so the result was a
segfault.
This patch fixes the problem by initializing strbuffer to an empty
string.
String buffer (strbuffer) is an object that resizes automatically when
data is added to it. It was implemented by generalizing the technique
used in json_dumps().
