Test input exists check

test/fixtures/psbt.json

@@ -1,6 +1,20 @@
 {
   "signInput": {
     "checks": [
+      {
+        "description": "checks the input exists",
+        "shouldSign": {
+          "psbt": "cHNidP8BADMBAAAAAXVa+rWvBGNyifYXEMlTten9+qC0xuHcAMxQYrQTwX1dAAAAAAD/////AAAAAAAAAQDAAgAAAAH//////////////////////////////////////////wAAAABrSDBFAiEAjtCPUj0vx3I5HFQKAUWHN0vCnT17jd41/omb4nobq/sCIAilSeQVi4mqykgBbs+Wz6PyqdMThi2gT463v4kPWk6cASECaSihTgej6zyYUQLWkPnBx68mOUGCIuXcWbZDMArbhWH/////AQDh9QUAAAAAGXapFC8spHIOpiw9giaEPd5RGkMYvXRHiKwAAAAAAAA=",
+          "inputToCheck": 0,
+          "WIF": "KxnAnQh6UJBxLF8Weup77yn8tWhLHhDhnXeyJuzmmcZA5aRdMJni"
+        },
+        "shouldThrow": {
+          "errorMessage": "No input #1",
+          "psbt": "cHNidP8BADMBAAAAAXVa+rWvBGNyifYXEMlTten9+qC0xuHcAMxQYrQTwX1dAAAAAAD/////AAAAAAAAAQDAAgAAAAH//////////////////////////////////////////wAAAABrSDBFAiEAjtCPUj0vx3I5HFQKAUWHN0vCnT17jd41/omb4nobq/sCIAilSeQVi4mqykgBbs+Wz6PyqdMThi2gT463v4kPWk6cASECaSihTgej6zyYUQLWkPnBx68mOUGCIuXcWbZDMArbhWH/////AQDh9QUAAAAAGXapFC8spHIOpiw9giaEPd5RGkMYvXRHiKwAAAAAAAA=",
+          "inputToCheck": 1,
+          "WIF": "KxnAnQh6UJBxLF8Weup77yn8tWhLHhDhnXeyJuzmmcZA5aRdMJni"
+        }
+      },
       {
         "description": "checks privkey matches the input it's signing",
         "shouldSign": {