lib-index: Do not crash if log is missing

Fixes sigfault when index and index.log files are
corrupted.

src/lib-index/mail-index.c

@@ -17,6 +17,7 @@
 #include "mail-index-sync-private.h"
 #include "mail-index-modseq.h"
 #include "mail-transaction-log.h"
+#include "mail-transaction-log-view-private.h"
 #include "mail-cache.h"
 
 #include <stdio.h>
@@ -600,8 +601,16 @@ int mail_index_open(struct mail_index *index, enum mail_index_open_flags flags)
 		mail_index_close_nonopened(index);
 		return ret;
 	}
+
 	index->open_count++;
 
+	if (index->log->head == NULL) {
+		mail_index_close(index);
+		mail_index_set_error(index, "Index is corrupted "
+					    "(log->view->head == NULL)");
+		return -1;
+	}
+
 	i_assert(index->map != NULL);
 	mail_index_alloc_cache_index_opened(index);
 	return 1;

src/lib-index/mail-transaction-log-view.c

@@ -15,6 +15,8 @@ mail_transaction_log_view_open(struct mail_transaction_log *log)
 	view->log = log;
 	view->broken = TRUE;
 
+	i_assert(view->log->head != NULL);
+
 	view->head = view->tail = view->log->head;
 	view->head->refcount++;
 	i_array_init(&view->file_refs, 8);