[ffmpeg] Fix `--ffmpeg-location` when directory is given

Bug introduced in 89efdc15dd
Closes #654

yt_dlp/postprocessor/ffmpeg.py

@@ -109,7 +109,9 @@ class FFmpegPostProcessor(PostProcessor):
                         'Continuing without ffmpeg.' % (location))
                     self._versions = {}
                     return
-                elif not os.path.isdir(location):
+                elif os.path.isdir(location):
+                    dirname, basename = location, None
+                else:
                     basename = os.path.splitext(os.path.basename(location))[0]
                     basename = next((p for p in programs if basename.startswith(p)), 'ffmpeg')
                     dirname = os.path.dirname(os.path.abspath(location))
@@ -118,6 +120,7 @@ class FFmpegPostProcessor(PostProcessor):
 
                 self._paths = dict(
                     (p, os.path.join(dirname, p)) for p in programs)
+                if basename:
                     self._paths[basename] = location
                 self._versions = dict(
                     (p, get_ffmpeg_version(self._paths[p])) for p in programs)