jsonapi: fix memory leakage during OOM error recovery.

jsonapi: fix memory leakage during OOM error recovery.

Coverity pointed out that inc_lex_level() would leak memory
(not to mention corrupt the pstack data structure) if some
but not all of its three REALLOC's failed.  To fix, store
successfully-updated pointers back into the pstack struct
immediately.

Oversight in 0785d1b8b, so no need for back-patch.