rpc: fix potential getblock handling problem, add a test for it

It should only accept hex strings and numbers, anything else is wrong.

pkg/rpc/server.go

@@ -141,7 +141,7 @@ Methods:
 				break Methods
 			}
 			hash = s.chain.GetHeaderHash(num)
-		case defaultT:
+		default:
 			resultsErr = errInvalidParams
 			break Methods
 		}

pkg/rpc/server_test.go

@@ -126,6 +126,11 @@ var rpcTestCases = map[string][]rpcTestCase{
 			params: `[]`,
 			fail:   true,
 		},
+		{
+			name:   "bad params",
+			params: `[[]]`,
+			fail:   true,
+		},
 		{
 			name:   "invalid height",
 			params: `[-1]`,