[perl #40053] Empty PGE rule causes error. rule x { }

[Kevin Tew]

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It should die with a more informative error message and a line number or
rule name.

Null PMC access in find_method()
current instr.: 'parrot;PGE::P6Regex;compile_p6regex' pc 6355
(compilers/pge/PGE/P6Regex.pir:128)
called from Sub 'parrot;PGE::P6Grammar;regex_stmt' pc 626
(../../compilers/pge/pgc.pir:338)
called from Sub 'parrot;PGE::P6Grammar;compile_p6grammar' pc 345
(../../compilers/pge/pgc.pir:225)
called from Sub 'parrot;PGE::P6Grammar;main' pc 135
(../../compilers/pge/pgc.pir:111)