Error when running in debug mode

**Project dependencies**
- react-native 0.51.0
- react-native-obfuscating-transformer ^1.0.0-rc.0

**Problem details**
- Add two files with info shown in [Usage](https://github.com/javascript-obfuscator/react-native-obfuscating-transformer#usage)
- run react-native run-ios to start app in debug mode

**Observation**
Following error was thrown in console:

```
Bundling `index.js` [development, non-minified] 0.0% (0/952), failed.
error: bundling failed: Error: original.line and original.column are not numbers -- you probably meant to omit the original mapping entirely and only map the generated position. If so, pass null for the original mapping instead of an object with empty or null values.
at SourceMapGenerator_validateMapping [as _validateMapping] (/Users/{my_account}/Documents/{my_project}/node_modules/react-native-obfuscating-transformer/node_modules/source-map/lib/source-map-generator.js:276:15)
at SourceMapGenerator_addMapping [as addMapping] (/Users/{my_account}/Documents/{my_project}/node_modules/react-native-obfuscating-transformer/node_modules/source-map/lib/source-map-generator.js:110:12)
at /Users/{my_account}/Documents/{my_project}/node_modules/react-native-obfuscating-transformer/dist/composeSourceMaps.js:9:19
at Array.forEach ()
at Object.convertMetroRawSourceMapToStandardSourceMap (/Users/{my_account}/Documents/{my_project}/node_modules/react-native-obfuscating-transformer/dist/composeSourceMaps.js:7:9)
at Object.obfuscateCodePreservingSourceMap (/Users/{my_account}/Documents/{my_project}/node_modules/react-native-obfuscating-transformer/dist/obfuscateCode.js:17:35)
at Object.transform (/Users/{my_account}/Documents/{my_project}/node_modules/react-native-obfuscating-transformer/dist/obfuscatingTransformer.js:72:96)
at asyncify (/Users/{my_account}/Documents/{my_project}/node_modules/metro-bundler/src/JSTransformer/worker/index.js:114:35)
at /Users/{my_account}/Documents/{my_project}/node_modules/async/asyncify.js:82:27
```

Reproduce the failure with react-native run-ios in debug mode using the two files described in the Usage instructions. Start by reading dist/composeSourceMaps.js and the source-map validation error, then check the related obfuscateCode.js and obfuscatingTransformer.js flow. Done means the debug bundle completes without the source-map mapping error.

Scritto dal modello di indicizzazione a partire dal testo della issue.