repair: initialize row_level_repair: _zero_rows

Avoid following UBSAN error:
repair/row_level.cc:2141:7: runtime error: load of value 240, which is not a valid value for type 'bool'

Fixes #5531

Signed-off-by: Benny Halevy <bhalevy@scylladb.com>

repair/row_level.cc

@@ -2174,7 +2174,7 @@ class row_level_repair {
 
     // If the total size of the `_row_buf` on either of the nodes is zero,
     // we set this flag, which is an indication that rows are not synced.
-    bool _zero_rows;
+    bool _zero_rows = false;
 
     // Sum of estimated_partitions on all peers
     uint64_t _estimated_partitions = 0;