compression: Allocate memory for compressEnd

Before this change the code assumed that the destination temp. buffer
always had enough free space for LZ4F_compressEnd operation. This change
checks the free size before calling LZ4F_compressEnd and allocates a new
buffer if necessary.

Note that the free space required is computed using LZ4F_compressBound
with a srcSize of 0. By looking at LZ4F_compressEnd it may appear that
we can just check if 12 bytes are free in the destination buffer, whereas
LZ4F_compressBound always assumes at least 65443 ie 65535 + block header
size (4) + frame footer (4).

However, lz4frame docstring for LZ4F_compressBound states that:

"When srcSize==0, LZ4F_compressBound() provides an upper bound for
LZ4F_flush() and LZ4F_compressEnd() instead."

Similar directions are mentioned on LZ4 gh issues, so it is safer to be
conservative and possibly allocate the extra bytes.

src/v/compression/internal/lz4_frame_compressor.cc

@@ -145,6 +145,16 @@ iobuf lz4_frame_compressor::compress(const iobuf& b) {
         output_cursor += code;
     }
 
+    if (const auto sz_for_compress_end = LZ4F_compressBound(0, &prefs);
+        output_sz - output_cursor < sz_for_compress_end) {
+        obuf.trim(output_cursor);
+        ret.append(std::move(obuf));
+        obuf = ss::temporary_buffer<char>(sz_for_compress_end);
+        output = obuf.get_write();
+        output_sz = obuf.size();
+        output_cursor = 0;
+    }
+
     code = LZ4F_compressEnd(
       ctx, output + output_cursor, output_sz - output_cursor, nullptr);
     check_lz4_error("lz4f_compressend:{}", code);