DataFlash: prevent the dataflash erase problem

only allow writes to dataflash block devices once StartNewLog() has
been called

libraries/DataFlash/DataFlash_Block.cpp

@@ -37,7 +37,7 @@ void DataFlash_Block::FinishWrite(void)
 
 void DataFlash_Block::WriteBlock(const void *pBuffer, uint16_t size)
 {
-    if (!CardInserted()) {
+    if (!CardInserted() || !log_write_started) {
         return;
     }
     while (size > 0) {
@@ -163,7 +163,9 @@ void DataFlash_Block::EraseAll()
     hal.scheduler->delay(100);
     StartWrite(df_NumPages+1);
     uint32_t version = DF_LOGGING_FORMAT;
+    log_write_started = true;
     WriteBlock(&version, sizeof(version));
+    log_write_started = false;
     FinishWrite();
     hal.scheduler->delay(100);
 }
@@ -173,8 +175,9 @@ void DataFlash_Block::EraseAll()
  */
 bool DataFlash_Block::NeedErase(void)
 {
-    uint32_t version;
+    uint32_t version = 0;
     StartRead(df_NumPages+1);
     ReadBlock(&version, sizeof(version));
+    StartRead(1);
     return version != DF_LOGGING_FORMAT;
 }

libraries/DataFlash/DataFlash_Block.h

@@ -53,6 +53,7 @@ private:
     uint16_t df_Read_PageAdr;
     uint16_t df_FileNumber;
     uint16_t df_FilePage;
+    bool log_write_started;
 
     /*
       functions implemented by the board specific backends

libraries/DataFlash/LogFile.cpp

@@ -58,6 +58,7 @@ uint16_t DataFlash_Block::start_new_log(void)
         SetFileNumber(1);
         StartWrite(1);
         //Serial.println("start log from 0");
+        log_write_started = true;
         return 1;
     }
 
@@ -78,6 +79,7 @@ uint16_t DataFlash_Block::start_new_log(void)
         SetFileNumber(new_log_num);
         StartWrite(last_page + 1);
     }
+    log_write_started = true;
     return new_log_num;
 }