In impure push mode, don't allow more than one yypstate to be allocated

In impure push mode, don't allow more than one yypstate to be allocated
since multiple impure parsers would corrupt yynerrs.
* data/push.c (yypstate_allocated): New static global variable
initialized to 0.
(yypull_parse): If yypstate_new returns 0, don't report it as memory
exhaustion if yypstate_allocated is 1, but still return 2.
(yypstate_new): Invoke yyerror and return 0 if yypstate_allocated is
already 1.  Otherwise, set it to 1.
(yypstate_delete): Set it to 0.
* tests/push.at (Push Parsing: Multiple impure instances): New test
case.