Check return value of JS_NewStringCopyZ (for NULL)

Kind of a shot in the dark here: Max (WESTLINE) is reporting HEAP CORRUPTION
debug assertion in websrvr.dll. In the 2 instances reported, a long (336
char) JSON "query value" was logged by apparent spam-bot trying to create a
a new user account ("send-me-free-stuff" is one of the JSON properties).

JS_NewStringCopyZ() can return NULL in a low memory situation, though I don't
know that explains possible heap corruption.