doxygen: Escape '\' in Doxygen documentation.

(cherry picked from commit c81a2b9b4f)

libavformat/internal.h

@@ -157,14 +157,14 @@ void ff_put_v(AVIOContext *bc, uint64_t val);
 
 /**
  * Read a whole line of text from AVIOContext. Stop reading after reaching
- * either a \n, a \0 or EOF. The returned string is always \0 terminated,
+ * either a \\n, a \\0 or EOF. The returned string is always \\0-terminated,
  * and may be truncated if the buffer is too small.
  *
  * @param s the read-only AVIOContext
  * @param buf buffer to store the read line
  * @param maxlen size of the buffer
  * @return the length of the string written in the buffer, not including the
- *         final \0
+ *         final \\0
  */
 int ff_get_line(AVIOContext *s, char *buf, int maxlen);