Fix: uninitialized variable in lib_ring_buffer_reserve_committed

Fix: uninitialized variable in lib_ring_buffer_reserve_committed

This internal function implemented in libringbuffer is not used within
lttng-ust actually, but uses an uninitialized variable:

As reported by clang:

        ./frontend_internal.h:263:75: warning: variable 'idx' is uninitialized when used here [-Wuninitialized]
                struct commit_counters_hot *cc_hot = shmp_index(handle, buf->commit_hot, idx);
                                                                                         ^~~
        ./shm.h:74:86: note: expanded from macro 'shmp_index'
                        ____ptr_ret = (__typeof__(____ptr_ret)) _shmp_offset((handle)->table, &(ref)._ref, index, sizeof(*____ptr_ret));        \
                                                                                                           ^~~~~
        ./frontend_internal.h:262:27: note: initialize the variable 'idx' to silence this warning
                unsigned long offset, idx, commit_count;
                                         ^
                                          = 0
        In file included from ring_buffer_backend.c:29:
        In file included from ./backend.h:33:
        ./frontend_internal.h:263:75: warning: variable 'idx' is uninitialized when used here [-Wuninitialized]
                struct commit_counters_hot *cc_hot = shmp_index(handle, buf->commit_hot, idx);
                                                                                         ^~~
        ./shm.h:74:86: note: expanded from macro 'shmp_index'
                        ____ptr_ret = (__typeof__(____ptr_ret)) _shmp_offset((handle)->table, &(ref)._ref, index, sizeof(*____ptr_ret));        \
                                                                                                           ^~~~~
        ./frontend_internal.h:262:27: note: initialize the variable 'idx' to silence this warning
                unsigned long offset, idx, commit_count;
                                         ^
                                          = 0

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>