sstables: compressed_file_data_source_impl: get: throw malformed_ssta…

…ble_exception on premature eof

Currently, the reader might dereference a null pointer
if the input stream reaches eof prematurely,
and read_exactly returns an empty temporary_buffer.

Detect this condition before dereferencing the buffer
and sstables::malformed_sstable_exception.

Fixes scylladb#13599

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

sstables/compress.cc

@@ -389,6 +389,9 @@ class compressed_file_data_source_impl : public data_source_impl {
             throw std::runtime_error("compressed reader out of sync");
         }
         return _input_stream->read_exactly(addr.chunk_len).then([this, addr](temporary_buffer<char> buf) {
+            if (!buf.size()) {
+                throw sstables::malformed_sstable_exception(format("compressed reader hit premature end-of-file at file offset {}", addr.chunk_len, _underlying_pos));
+            }
             return _permit.request_memory(_compression_metadata->uncompressed_chunk_length()).then(
                     [this, addr, buf = std::move(buf)] (reader_permit::resource_units res_units) mutable {
                 // The last 4 bytes of the chunk are the adler32/crc32 checksum