img_optimize_png.py not parsing temp path correctly

[Tenfleques]

Traceback (most recent call last):
File "/usr/local/lib/python3.7/site-packages/optimize_images/img_optimize_png.py", line 125, in optimize_png
img.save(temp_file_path, optimize=True, format=result_format)
File "/usr/local/lib/python3.7/site-packages/PIL/Image.py", line 1966, in save
fp = builtins.open(filename, "w+b")
PermissionError: [Errno 13] Permission denied: '/tempwoocommerce-placeholder.png'

[Tenfleques]

I copied this control from img_optimize_jpg.py to img_optimize_png.py

if folder == '':
       folder = os.getcwd()

but I think you need to find out why os.path.split(t.src_path) is not giving you the folder pjath

[victordomingos]

Just a quick notice to let you know that, on the latest commit (not yet available in PyPI), I have implemented in-memory buffers using BytesIO, instead of in-disk temporary files.