(YYSIZE_MAXIMUM): New macro.

(YYSIZE_MAXIMUM): New macro.
(YYSTACK_ALLOC_MAXIMUM): Use it.
(yysyntax_error): New function.
(yyparse) [YYERROR_VERBOSE]: Don't leak memory indefinitely if
multiple syntax errors are reported, and alloca is being used.
Instead, reallocate buffers twice as big each time, so that
we waste at most half the allocated memory.  Start with a small
(128-byte) buffer that will suffice in most cases anyway.
Use yysyntax_error to do most of the work.