在ubuntu1804中使用ffmepg进行图片格式转换时，无法对.png格式进行转换
输入语句：ffmepg -i wxh.png out.gif
运行结果：png_ pipe @ 0x55a2b6c80040] could not find codec parameters for stream 0 (Video: png, none): unspecified sizeConsider increasing the value for the ' analyzeduration' and ' probesize' options
Input #0, png_ pipe, from ' wxh.png' :
Duration: N/A, bitrate: N/A
stream #0:0: Video: png, none, 25 tbr, 25 tbn, 25 tbc
Stream mapping:
Stream #0:0-> #0:0 (? (?) -> gif (native))
Decoder ( codec png)_ not found for input stream #0 :0