signextend removes two stack items

Author: cdetrio

lib/opcodes.js

@@ -12,7 +12,7 @@ const codes = {
   0x08: ['ADDMOD', 8, 3, 1, false],
   0x09: ['MULMOD', 8, 3, 1, false],
   0x0a: ['EXP', 10, 2, 1, false],
-  0x0b: ['SIGNEXTEND', 5, 1, 1, false],
+  0x0b: ['SIGNEXTEND', 5, 2, 1, false],
 
   // 0x10 range - bit ops
   0x10: ['LT', 3, 2, 1, false],