Retry still image without skipping frames on failure

Some videos can't be seeked, either because they are too short or
because the format doesn't support it. In this case, just take the very
first frame as still image.

Fixes thumbsup/thumbsup#76

lib/index.js

@@ -1,4 +1,5 @@
 const async = require('async')
+const fs = require('fs')
 const gm = require('gm')
 const mkdirp = require('mkdirp')
 const path = require('path')
@@ -100,6 +101,12 @@ exports.still = function (source, target, options, callback) {
 }
 
 function extractFrame (source, target, callback) {
-  const args = ['-ss', '0.1', '-i', source, '-vframes', 1, '-y', target]
-  ffmpeg.exec(args, callback)
+  const args = ['-i', source, '-vframes', 1, '-y', target]
+  ffmpeg.exec(['-ss', '0.1'].concat(args), (err) => {
+    if (fs.existsSync(target)) {
+      callback(err)
+    } else {
+      ffmpeg.exec(args, callback)
+    }
+  })
 }

test/still.js

@@ -17,6 +17,14 @@ tape('extracts a frame from a very short video', (test) => {
   })
 })
 
+tape('extracts a frame from a video that only has a single frame', (test) => {
+  diff.still(test, {
+    input: 'videos/single-frame.mov',
+    expect: 'videos/single-frame.jpg',
+    options: { height: 150 }
+  })
+})
+
 tape('extracts and crops a frame from an MP4 video', (test) => {
   diff.still(test, {
     input: 'videos/countdown.mp4',