flat_mutation_reader: Move field initialization to initializer list

This works around a problem of std::terminate() being called in debug
mode build if initialization of _current throws.

Backtrace:

Thread 2 "row_cache_test_" received signal SIGABRT, Aborted.
0x00007ffff17ce9fb in raise () from /lib64/libc.so.6
(gdb) bt
  #0  0x00007ffff17ce9fb in raise () from /lib64/libc.so.6
  #1  0x00007ffff17d077d in abort () from /lib64/libc.so.6
  #2  0x00007ffff5773025 in __gnu_cxx::__verbose_terminate_handler() () from /lib64/libstdc++.so.6
  #3  0x00007ffff5770c16 in ?? () from /lib64/libstdc++.so.6
  #4  0x00007ffff576fb19 in ?? () from /lib64/libstdc++.so.6
  #5  0x00007ffff5770508 in __gxx_personality_v0 () from /lib64/libstdc++.so.6
  #6  0x00007ffff3ce4ee3 in ?? () from /lib64/libgcc_s.so.1
  #7  0x00007ffff3ce570e in _Unwind_Resume () from /lib64/libgcc_s.so.1
  #8  0x0000000003633602 in reader::reader (this=0x60e0001160c0, r=...) at flat_mutation_reader.cc:214
  #9  0x0000000003655864 in std::make_unique<make_forwardable(flat_mutation_reader)::reader, flat_mutation_reader>(flat_mutation_reader &&) (__args#0=...)
    at /usr/include/c++/7/bits/unique_ptr.h:825
  #10 0x0000000003649a63 in make_flat_mutation_reader<make_forwardable(flat_mutation_reader)::reader, flat_mutation_reader>(flat_mutation_reader &&) (args#0=...)
    at flat_mutation_reader.hh:440
  #11 0x000000000363565d in make_forwardable (m=...) at flat_mutation_reader.cc:270
  #12 0x000000000303f962 in memtable::make_flat_reader (this=0x61300001d540, s=..., range=..., slice=..., pc=..., trace_state_ptr=..., fwd=..., fwd_mr=...)
    at memtable.cc:592

Message-Id: <1528792447-13336-1-git-send-email-tgrabiec@scylladb.com>
(cherry picked from commit 6d6b93d)

flat_mutation_reader.cc

@@ -183,10 +183,7 @@ flat_mutation_reader make_delegating_reader(flat_mutation_reader& r) {
 flat_mutation_reader make_forwardable(flat_mutation_reader m) {
     class reader : public flat_mutation_reader::impl {
         flat_mutation_reader _underlying;
-        position_range _current = {
-            position_in_partition(position_in_partition::partition_start_tag_t()),
-            position_in_partition(position_in_partition::after_static_row_tag_t())
-        };
+        position_range _current;
         mutation_fragment_opt _next;
         // When resolves, _next is engaged or _end_of_stream is set.
         future<> ensure_next() {
@@ -201,7 +198,10 @@ flat_mutation_reader make_forwardable(flat_mutation_reader m) {
             });
         }
     public:
-        reader(flat_mutation_reader r) : impl(r.schema()), _underlying(std::move(r)) { }
+        reader(flat_mutation_reader r) : impl(r.schema()), _underlying(std::move(r)), _current({
+            position_in_partition(position_in_partition::partition_start_tag_t()),
+            position_in_partition(position_in_partition::after_static_row_tag_t())
+        }) { }
         virtual future<> fill_buffer(db::timeout_clock::time_point timeout) override {
             return repeat([this] {
                 if (is_buffer_full()) {