lavf/sdp: Change pointer to configuration from char* to uint8_t*.

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

libavformat/sdp.c

@@ -347,7 +347,8 @@ static char *extradata2config(AVFormatContext *s, AVCodecParameters *par)
 
 static char *xiph_extradata2config(AVFormatContext *s, AVCodecParameters *par)
 {
-    char *config, *encoded_config;
+    uint8_t *config;
+    char *encoded_config;
     const uint8_t *header_start[3];
     int headers_len, header_len[3], config_len;
     int first_header_size;