openni2src: Fix unitialized variable compiler warning

openni2src: Fix unitialized variable compiler warning

gstopenni2src.cpp:721:14: error: variable 'oni_ts' is used uninitialized whenever 'if' condition is false [-Werror,-Wsometimes-uninitialized]
  } else if (src->color->isValid () && src->sourcetype == SOURCETYPE_COLOR) {
             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
gstopenni2src.cpp:752:26: note: uninitialized use occurs here
  GST_BUFFER_PTS (buf) = oni_ts - src->oni_start_ts;
                         ^~~~~~
gstopenni2src.cpp:721:10: note: remove the 'if' if its condition is always true
  } else if (src->color->isValid () && src->sourcetype == SOURCETYPE_COLOR) {
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
gstopenni2src.cpp:721:14: error: variable 'oni_ts' is used uninitialized whenever '&&' condition is false [-Werror,-Wsometimes-uninitialized]
  } else if (src->color->isValid () && src->sourcetype == SOURCETYPE_COLOR) {
             ^~~~~~~~~~~~~~~~~~~~~~
gstopenni2src.cpp:752:26: note: uninitialized use occurs here
  GST_BUFFER_PTS (buf) = oni_ts - src->oni_start_ts;
                         ^~~~~~
gstopenni2src.cpp:721:14: note: remove the '&&' if its condition is always true
  } else if (src->color->isValid () && src->sourcetype == SOURCETYPE_COLOR) {
             ^~~~~~~~~~~~~~~~~~~~~~~~~
gstopenni2src.cpp:642:18: note: initialize the variable 'oni_ts' to silence this warning
  uint64_t oni_ts;
                 ^
                  = 0