dbg/disasm: max opcode length

[a/b]

od-win32/debug_win32.cpp, line 583 (GetPrevAddr() function):

	dasmaddr = addr - 20;

This should be 22 since the longest opcode is 22 bytes. For example, if you have something like:

	move.l	([$87654321.l,a0],$12345678.l),([$87654321.l,a0],$12345678.l)

and scroll one line up in debugger (UI version, so F5 and then alt+up) it won't properly detect the whole opcode, and it will end up somewhere in the middle of it since it only goes back 20 bytes.