journal-remote: set a limit on the number of fields in a message

Existing use of E2BIG is replaced with ENOBUFS (entry too long), and E2BIG is
reused for the new error condition (too many fields).

This matches the change done for systemd-journald, hence forming the second
part of the fix for CVE-2018-16865
(https://bugzilla.redhat.com/show_bug.cgi?id=1653861).

src/shared/journal-importer.c

@@ -23,6 +23,9 @@ enum {
 };
 
 static int iovw_put(struct iovec_wrapper *iovw, void* data, size_t len) {
+        if (iovw->count >= ENTRY_FIELD_COUNT_MAX)
+                return -E2BIG;
+
         if (!GREEDY_REALLOC(iovw->iovec, iovw->size_bytes, iovw->count + 1))
                 return log_oom();
 
@@ -97,7 +100,7 @@ static int get_line(JournalImporter *imp, char **line, size_t *size) {
 
                 imp->scanned = imp->filled;
                 if (imp->scanned >= DATA_SIZE_MAX)
-                        return log_error_errno(SYNTHETIC_ERRNO(E2BIG),
+                        return log_error_errno(SYNTHETIC_ERRNO(ENOBUFS),
                                                "Entry is bigger than %u bytes.",
                                                DATA_SIZE_MAX);