closedcaption: fix build error in OSX

closedcaption: fix build error in OSX

Fixes the following error.

gstccconverter.c:677:7: error: variable 'len' is used uninitialized whenever 'if' condition is false
      [-Werror,-Wsometimes-uninitialized]
  if (flags & 0x40) {
      ^~~~~~~~~~~~
gstccconverter.c:698:10: note: uninitialized use occurs here
  return len;
         ^~~
gstccconverter.c:677:3: note: remove the 'if' if its condition is always true
  if (flags & 0x40) {
  ^~~~~~~~~~~~~~~~~~
gstccconverter.c:572:12: note: initialize the variable 'len' to silence this warning
  guint len;
           ^
            = 0