simplify CheckMinimalPush checks, add safety assert

src/script/interpreter.cpp

@@ -220,15 +220,17 @@ bool static CheckPubKeyEncoding(const valtype &vchPubKey, unsigned int flags, co
 }
 
 bool static CheckMinimalPush(const valtype& data, opcodetype opcode) {
+    // Excludes OP_1NEGATE, OP_1-16 since they are by definition minimal
+    assert(0 <= opcode && opcode <= OP_PUSHDATA4);
     if (data.size() == 0) {
         // Could have used OP_0.
         return opcode == OP_0;
     } else if (data.size() == 1 && data[0] >= 1 && data[0] <= 16) {
         // Could have used OP_1 .. OP_16.
-        return opcode == OP_1 + (data[0] - 1);
+        return false;
     } else if (data.size() == 1 && data[0] == 0x81) {
         // Could have used OP_1NEGATE.
-        return opcode == OP_1NEGATE;
+        return false;
     } else if (data.size() <= 75) {
         // Could have used a direct push (opcode indicating number of bytes pushed + those bytes).
         return opcode == data.size();