This is also what av_base64_encode() expects.
Fixes the following warnings with clang:
libavformat/sdp.c:394:40: warning: implicit conversion from 'int' to 'char' changes value from 254 to -2
libavformat/sdp.c:395:40: warning: implicit conversion from 'int' to 'char' changes value from 205 to -51
libavformat/sdp.c:396:40: warning: implicit conversion from 'int' to 'char' changes value from 186 to -70