Add warn_unused_result to strbuffer_init.

This adds a compiler warning when strbuffer_init return value is
ignored.  unpack_object is updated to deal with errors produced
while building unrecognized_keys.

src/strbuffer.h

@@ -16,7 +16,7 @@ typedef struct {
     size_t size;     /* bytes allocated */
 } strbuffer_t;
 
-int strbuffer_init(strbuffer_t *strbuff);
+int strbuffer_init(strbuffer_t *strbuff) JANSSON_ATTRS(warn_unused_result);
 void strbuffer_close(strbuffer_t *strbuff);
 
 void strbuffer_clear(strbuffer_t *strbuff);