[core] speed up the clearing of MBR/GPT sectors

* write_sector() should really only be used when writing single sectors as it
  is way to slow for anything else => Switch to using WriteFileWithRetry().
* Also revert an unwarranted change from f0047986e7.

src/stdio.c

@@ -855,6 +855,10 @@ BOOL WriteFileWithRetry(HANDLE hFile, LPCVOID lpBuffer, DWORD nNumberOfBytesToWr
 	DWORD nTry;
 	BOOL readFilePointer;
 	LARGE_INTEGER liFilePointer, liZero = { { 0,0 } };
+	DWORD NumberOfBytesWritten;
+
+	if (lpNumberOfBytesWritten == NULL)
+		lpNumberOfBytesWritten = &NumberOfBytesWritten;
 
 	// Need to get the current file pointer in case we need to retry
 	readFilePointer = SetFilePointerEx(hFile, liZero, &liFilePointer, FILE_CURRENT);