Fix no-prototype-builtins error

This fix doesn't seem to break anything, though I'll admit it's straight off stackoverflow.

lib/file-helper.js

@@ -72,26 +72,22 @@ exports.getFingerprint = function (file) {
   // We only know the path of the current file when we're compiling the layout –
   // calls to this function with a relative path will fail if made from the
   // source files themselves.
-  if (filePath) {
-    // Use path.join to correctly join, but metalsmith-fingerprint-ignore
-    // always expects forward slashes, so replace any backslashes (Windows)
-    // with a forward slashes.
-    const relativeFile = path.join(filePath, file).replace(/\\/g, '/')
-
-    if (fingerprints.hasOwnProperty(relativeFile)) {
-      return '/' + fingerprints[relativeFile]
-    }
-  }
-
-  // Look for a fingerprinted asset at this path relative to the site root
-  if (fingerprints.hasOwnProperty(file)) {
-    return '/' + fingerprints[file]
+  if (!fingerprints[file] && filePath) {
+  // Use path.join to correctly join, but metalsmith-fingerprint-ignore
+  // always expects forward slashes, so replace any backslashes (Windows)
+  // with a forward slashes.
+    file = path.join(filePath, file).replace(/\\/g, '/')
   }
 
   // The thrown error will stop the build, but not provide any useful output,
   // so we have to console.log as well.
-  console.log(`Could not find fingerprint for file ${file}`)
-  throw new Error(`Could not find fingerprint for file ${file}`)
+  if (!fingerprints[file]) {
+    console.log(`Could not find fingerprint for file ${file}`)
+    throw new Error(`Could not find fingerprint for file ${file}`)
+  }
+
+  // Look for a fingerprinted asset at this path relative to the site root
+  return '/' + fingerprints[file]
 }
 
 // This helper function takes a path of a *.md.njk file and