multimedia/mp4v2: unbreak build with Clang 6 (C++14 by default)

src/mp4.cpp:679:20: error: cannot initialize return object of type 'mp4v2_ismacrypParams *' (aka 'mp4v2_ismacryp_session_params *') with an rvalue of type 'MP4TrackId' (aka 'unsigned int')

return MP4_INVALID_TRACK_ID;
       ^~~~~~~~~~~~~~~~~~~~

./include/mp4v2/general.h:45:33: note: expanded from macro 'MP4_INVALID_TRACK_ID'
#define MP4_INVALID_TRACK_ID ((MP4TrackId)0) /**< Constant: invalid MP4TrackId. */

^~~~~~~~~~~~~~~

src/rtphint.cpp:348:32: error: comparison between pointer and integer ('const char *' and 'int')

if (pSlash != '\0') {
    ~~~~~~ ^  ~~~~

util/mp4chaps.cpp:712:14: error: case value evaluates to 4026531855, which cannot be narrowed to type 'int'

[-Wc++11-narrowing]
  case LC_CHP_REMOVE:
       ^

util/mp4file.cpp:206:14: error: case value evaluates to 4026531848, which cannot be narrowed to type 'int'

[-Wc++11-narrowing]
  case LC_DUMP:
       ^

util/mp4art.cpp:413:14: error: case value evaluates to 4026531852, which cannot be narrowed to type 'int'

[-Wc++11-narrowing]
  case LC_EXTRACT:
       ^

libutil/Utility.cpp:544:18: error: case value evaluates to 4026531842, which cannot be narrowed to type 'int'

[-Wc++11-narrowing]
      case LC_VERBOSE:
           ^

Reported by: antoine (via bug 224669)