Fix a crash when decoding messages that are too short

src/message.cpp

@@ -49,6 +49,9 @@ T varint_decode(
     std::uint8_t const * varint_end
 ) {
     T value = 0;
+    if (varint_end == varint_start) {
+        return 0;
+    }
     do {
         value <<= 7;
         value |= 0x7F & *(--varint_end);

tests/test_olm_decrypt.cpp

@@ -0,0 +1,69 @@
+#include "olm/olm.hh"
+#include "unittest.hh"
+
+const char * test_cases[] = {
+    "41776f",
+    "7fff6f0101346d671201"
+};
+
+
+const char * session_data =
+    "E0p44KO2y2pzp9FIjv0rud2wIvWDi2dx367kP4Fz/9JCMrH+aG369HGymkFtk0+PINTLB9lQRt"
+    "ohea5d7G/UXQx3r5y4IWuyh1xaRnojEZQ9a5HRZSNtvmZ9NY1f1gutYa4UtcZcbvczN8b/5Bqg"
+    "e16cPUH1v62JKLlhoAJwRkH1wU6fbyOudERg5gdXA971btR+Q2V8GKbVbO5fGKL5phmEPVXyMs"
+    "rfjLdzQrgjOTxN8Pf6iuP+WFPvfnR9lDmNCFxJUVAdLIMnLuAdxf1TGcS+zzCzEE8btIZ99mHF"
+    "dGvPXeH8qLeNZA";
+
+void decode_hex(
+    const char * input,
+    std::uint8_t * output, std::size_t output_length
+) {
+    std::uint8_t * end = output + output_length;
+    while (output != end) {
+        char high = *(input++);
+        char low = *(input++);
+        if (high >= 'a') high -= 'a' - ('9' + 1);
+        if (low >= 'a') low -= 'a' - ('9' + 1);
+        uint8_t value = ((high - '0') << 4) | (low - '0');
+        *(output++) = value;
+    }
+}
+
+void decrypt_case(int message_type, const char * test_case) {
+    std::uint8_t session_memory[olm_session_size()];
+    ::OlmSession * session = ::olm_session(session_memory);
+
+    std::uint8_t pickled[strlen(session_data)];
+    ::memcpy(pickled, session_data, sizeof(pickled));
+    ::olm_unpickle_session(session, "", 0, pickled, sizeof(pickled));
+
+    std::size_t message_length = strlen(test_case) / 2;
+    std::uint8_t message[message_length];
+    decode_hex(test_case, message, message_length);
+
+    size_t max_length = olm_decrypt_max_plaintext_length(
+        session, message_type, message, message_length
+    );
+
+    if (max_length == std::size_t(-1)) return;
+
+    uint8_t plaintext[max_length];
+    decode_hex(test_case, message, message_length);
+    olm_decrypt(
+        session, message_type,
+        message, message_length,
+        plaintext, max_length
+    );
+}
+
+
+int main() {
+{
+TestCase("Olm decrypt test");
+
+for (int i = 0; i < sizeof(test_cases)/ sizeof(const char *); ++i) {
+    decrypt_case(0, test_cases[i]);
+}
+
+}
+}