java: Add opcode assertion to verify_method

The verifier ignores opcodes it does not know about. However, if an
opcode is larger than the maximum defined opcode, verify_method() enters
an infinite loop because size of the instruction is zero.

Add an assertion to fail fast if that happens.

Signed-off-by: Pekka Enberg <penberg@iki.fi>
java/verify: tableswitch and lookupswitch parsing

Signed-off-by: Pekka Enberg <penberg@iki.fi>