Fix json_loadf and json_loadfd with empty input

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.

src/strbuffer.h

@@ -7,7 +7,7 @@ typedef struct {
     int size;
 } strbuffer_t;
 
-void strbuffer_init(strbuffer_t *strbuff);
+int strbuffer_init(strbuffer_t *strbuff);
 void strbuffer_close(strbuffer_t *strbuff);
 
 const char *strbuffer_value(strbuffer_t *strbuff);