Conditionals must not straddle two scripts.

Found by tests dhill is working on. We checked that ifs were closed at the end
of execution but not at script switching time, we now move this to just after
finishing a single script.

script.go

@@ -583,10 +583,6 @@ func (s *Script) CheckErrorCondition() (err error) {
 		}))
 		err = StackErrScriptFailed
 	}
-	if err == nil && len(s.condStack) != 1 {
-		// conditional execution stack context left active
-		err = StackErrMissingEndif
-	}
 	return err
 }
 
@@ -625,6 +621,11 @@ func (m *Script) Step() (done bool, err error) {
 	// prepare for next instruction
 	m.scriptoff++
 	if m.scriptoff >= len(m.scripts[m.scriptidx]) {
+		// Illegal to have an `if' that straddles two scripts.
+		if err == nil && len(m.condStack) != 1 {
+			return false, StackErrMissingEndif
+		}
+
 		m.numOps = 0 // number of ops is per script.
 		m.scriptoff = 0
 		if m.scriptidx == 0 && m.bip16 {