unpickle_account: Add error checking to the harness.

fuzzing/fuzzers/fuzz_unpickle_account.cpp

-#include "olm/account.hh"
 #include "fuzzing.hh"
+#include "olm/account.hh"
+#include "olm/olm.h"
+
+size_t fuzz_unpickle_account(
+    OlmAccount * account, void * pickled, size_t pickled_length
+) {
+    olm::Account & object = *reinterpret_cast<olm::Account *>(account);
+    std::uint8_t * const pos = reinterpret_cast<std::uint8_t *>(pickled);
+    std::uint8_t * const end = pos + pickled_length;
 
-int main(int argc, const char *argv[]) {
+    /* On success unpickle will return (pos + raw_length). If unpickling
+     * terminates too soon then it will return a pointer before
+     * (pos + raw_length). On error unpickle will return (pos + raw_length + 1).
+     */
+    if (end != unpickle(pos, end + 1, object)) {
+        if (object.last_error == OlmErrorCode::OLM_SUCCESS) {
+            object.last_error = OlmErrorCode::OLM_CORRUPTED_PICKLE;
+        }
+        return std::size_t(-1);
+    }
+
+    return pickled_length;
+}
+
+int main(int argc, const char * argv[]) {
     int pickle_fd = STDIN_FILENO;
     uint8_t * pickle_buffer;
     ssize_t pickle_length = check_errno(
-        "Error reading pickle file", read_file(pickle_fd, &pickle_buffer)
-    );
-    olm::Account * account = new olm::Account;
-    unpickle(pickle_buffer, pickle_buffer + pickle_length, *account);
+        "Error reading pickle file", read_file(pickle_fd, &pickle_buffer));
+
+    void * account_buf = malloc(olm_account_size());
+    if (!account_buf) {
+        return 3;
+    }
+    OlmAccount * account = olm_account(account_buf);
+
+    check_error(olm_account_last_error, account, "Error unpickling account",
+        fuzz_unpickle_account(account, pickle_buffer, pickle_length));
+
     free(pickle_buffer);
-    delete account;
+    free(account);
 
     return EXIT_SUCCESS;
 }