Fix problem when reading bz2 files

This fixes a case where libz2 tells us that there is no data any more
(BZ_STREAM_END), but we are not at the end of file, so there could be
more data. But then there isn't (nunused==0). It seems this doesn't
happen very often, but it does happen occasionally.

Fixes #373

Author: joto

include/osmium/io/bzip2_compression.hpp

@@ -294,22 +294,26 @@ namespace osmium {
                         detail::throw_bzip2_error(m_bzfile, "read failed", bzerror);
                     }
                     if (bzerror == BZ_STREAM_END) {
-                        void* unused = nullptr;
-                        int nunused = 0;
                         if (!feof(m_file.file())) {
+                            void* unused = nullptr;
+                            int nunused = 0;
                             ::BZ2_bzReadGetUnused(&bzerror, m_bzfile, &unused, &nunused);
                             if (bzerror != BZ_OK) {
                                 detail::throw_bzip2_error(m_bzfile, "get unused failed", bzerror);
                             }
-                            std::string unused_data{static_cast<const char*>(unused), static_cast<std::string::size_type>(nunused)};
-                            ::BZ2_bzReadClose(&bzerror, m_bzfile);
-                            if (bzerror != BZ_OK) {
-                                throw bzip2_error{"bzip2 error: read close failed", bzerror};
-                            }
-                            assert(unused_data.size() < std::numeric_limits<int>::max());
-                            m_bzfile = ::BZ2_bzReadOpen(&bzerror, m_file.file(), 0, 0, &*unused_data.begin(), static_cast<int>(unused_data.size()));
-                            if (!m_bzfile) {
-                                throw bzip2_error{"bzip2 error: read open failed", bzerror};
+                            if (nunused != 0) {
+                                std::string unused_data{static_cast<const char*>(unused), static_cast<std::string::size_type>(nunused)};
+                                ::BZ2_bzReadClose(&bzerror, m_bzfile);
+                                if (bzerror != BZ_OK) {
+                                    throw bzip2_error{"bzip2 error: read close failed", bzerror};
+                                }
+                                assert(unused_data.size() < std::numeric_limits<int>::max());
+                                m_bzfile = ::BZ2_bzReadOpen(&bzerror, m_file.file(), 0, 0, &*unused_data.begin(), static_cast<int>(unused_data.size()));
+                                if (!m_bzfile) {
+                                    throw bzip2_error{"bzip2 error: read open failed", bzerror};
+                                }
+                            } else {
+                                m_stream_end = true;
                             }
                         } else {
                             m_stream_end = true;