[doc] small spelling update

[added]: debugwarn for bad signature-size value

README.md

@@ -224,14 +224,14 @@ Default to `undefined`.
 
 `restrict` - *Boolean*
 
-**To be deprecated, see `signature-flags`.** 
+**To be deprecated, see `signature-flags`.**
 Restrict dyld loading. See doc about this [code signature flag](https://developer.apple.com/documentation/security/seccodesignatureflags/kseccodesignaturerestrict?language=objc) for more details. Disabled by default.
 
 `signature-flags` - *String*
 Comma separated string or array for [code signature flag](https://developer.apple.com/documentation/security/seccodesignatureflags?language=objc). Default to `undefined`.
 
 `signature-size` - *Number*
-Provide a value to be passed to `codesign` along with the `--signature-size` flag, to work around the *Signature too large to embed* issue. A value of `12000` should do it - see the [FAQ](https://github.com/electron/electron-osx-sign/wiki/FAQ) for details. Default to `undefined`.
+Provide a value to be passed to `codesign` along with the `--signature-size` flag, to work around the *signature too large to embed* issue. A value of `12000` should do it - see the [FAQ](https://github.com/electron/electron-osx-sign/wiki/FAQ) for details. Default to `undefined`.
 
 `strict-verify` - *Boolean|String|Array.<String>*
 

sign.js

@@ -158,8 +158,12 @@ function signApplicationAsync (opts) {
       } else {
         args.push('--timestamp')
       }
-      if (opts['signature-size'] && Number.isInteger(opts['signature-size'])) {
-        args.push('--signature-size', opts['signature-size'])
+      if (opts['signature-size']) {
+        if(Number.isInteger(opts['signature-size']) && opts['signature-size'] > 0) {
+          args.push('--signature-size', opts['signature-size'])
+        } else {
+          debugwarn(`Invalid value provided for --signature-size (${opts['signature-size']}). Must be a positive integer.`)
+        }
       }
 
       let optionsArguments = []