Avoid attempted reads past end of program image

Previously if what could be a load, store, branch, or call instruction
was located within last two bytes of the program image being
disassembled, the disassembler would try to read out of bounds to get
its address parameter. This change marks such cases as not being valid
instructions.

thingamajig_disasm.py

@@ -45,6 +45,7 @@ def segment(binary, origin):
 		valid = True
 		if not opcodes[opcode].rx and rx != 0: valid = False
 		if not opcodes[opcode].ry and ry != 0: valid = False
+		if opcodes[opcode].addr and ip + 2 >= len(binary): valid = False
 
 		if not valid:
 			raw = binary[ip:ip + 1]