ignore mapping if struct in memory (#1308)

fix https://github.com/ethereum/remix-project/issues/271

libs/remix-debug/src/solidity-decoder/types/Mapping.ts

@@ -38,7 +38,7 @@ export class Mapping extends RefType {
   decodeFromMemoryInternal (offset, memory) {
     // mappings can only exist in storage and not in memory
     // so this should never be called
-    return { value: '<not implemented>', length: '0x', type: this.typeName }
+    return { value: '', length: '0x0', type: this.typeName }
   }
 
   async decodeMappingsLocation (preimages, location, storageResolver) {

libs/remix-debug/src/solidity-decoder/types/Struct.ts

@@ -1,6 +1,7 @@
 'use strict'
 import { add } from './util'
 import { RefType } from './RefType'
+import { Mapping } from './Mapping'
 
 export class Struct extends RefType {
   members
@@ -33,7 +34,7 @@ export class Struct extends RefType {
       var contentOffset = offset
       var member = item.type.decodeFromMemory(contentOffset, memory)
       ret[item.name] = member
-      offset += 32
+      if (!(item.type instanceof Mapping)) offset += 32
     })
     return { value: ret, type: this.typeName }
   }