Make av_base64_encode() do not require the user to provide an

overallocated buffer where to put the encoded string.

See the thread:
"[PATCH] Improve documentation for libavutil/base64.h".

Originally committed as revision 17065 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavutil/base64.c

@@ -79,7 +79,7 @@ char *av_base64_encode(char * buf, int buf_len, const uint8_t * src, int len)
     int bytes_remaining = len;
 
     if (len >= UINT_MAX / 4 ||
-        buf_len < len * 4 / 3 + 12)
+        buf_len < (len+2) / 3 * 4 + 1)
         return NULL;
     ret = dst = buf;
     while (bytes_remaining) {