small fix of jsondump example for cases of realloc failures
small fix of jsondump example for cases of realloc failures

When realloc() function fails it returns NULL pointer.
But old data pointer remains valid in such a case.

It's a mistake to use old data pointer to store new
pointer returned by realloc. In case of realloc failure,
pointer is overwritten with NULL value, but old used memory
remains unreferenced and could not be even freed anymore.
Such mistakes could lead to memory leaks.

zserge/jsmn