DataFlash: stop logging when erasing logs

libraries/DataFlash/DataFlash_Block.cpp

@@ -156,6 +156,7 @@ uint16_t DataFlash_Block::GetFilePage()
 
 void DataFlash_Block::EraseAll()
 {
+    log_write_started = false;
     for (uint16_t j = 1; j <= (df_NumPages+1)/8; j++) {
         BlockErase(j);
         if (j%6 == 0) {

libraries/DataFlash/DataFlash_File.cpp

@@ -113,6 +113,7 @@ char *DataFlash_File::_lastlog_file_name(void)
 void DataFlash_File::EraseAll()
 {
     uint16_t log_num;
+    stop_logging();
     for (log_num=0; log_num<MAX_LOG_FILES; log_num++) {
         char *fname = _log_file_name(log_num);
         if (fname == NULL) {