probe incorrectly report video stream for mp3 files with poster image

[tahajahangir]

Here is a STREAM section of poster image for a mp3 file, reporter by ffprobe:

[STREAM]
index=1
codec_name=mjpeg
codec_long_name=MJPEG (Motion JPEG)
profile=unknown
codec_type=video
codec_time_base=1/90000
codec_tag_string=[0][0][0][0]
codec_tag=0x0000
width=0
height=0
has_b_frames=0
sample_aspect_ratio=0:1
display_aspect_ratio=0:0
pix_fmt=unknown
level=-99
timecode=N/A
id=N/A
r_frame_rate=90000/1
avg_frame_rate=0/0
time_base=1/90000
start_pts=N/A
start_time=N/A
duration_ts=26818560
duration=297.984000
bit_rate=N/A
nb_frames=N/A
nb_read_frames=N/A
nb_read_packets=N/A
DISPOSITION:default=0
DISPOSITION:dub=0
DISPOSITION:original=0
DISPOSITION:comment=0
DISPOSITION:lyrics=0
DISPOSITION:karaoke=0
DISPOSITION:forced=0
DISPOSITION:hearing_impaired=0
DISPOSITION:visual_impaired=0
DISPOSITION:clean_effects=0
DISPOSITION:attached_pic=1
TAG:title=
TAG:comment=Cover (front)
[/STREAM]

[tahajahangir]

Fixed in #28