openocd: manually remove NULL comparisons

For the remaining NULL comparisons, remove then manually.

While there, make more readable a loop, by moving the assigment
out of the loop condition.

Change-Id: I44193aaa95813156a3a79c16b80e1ad333dc1eaf
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/6353
Tested-by: jenkins

src/helper/fileio.c

@@ -208,7 +208,7 @@ int fileio_read_u32(struct fileio *fileio, uint32_t *data)
 
 static int fileio_local_fgets(struct fileio *fileio, size_t size, void *buffer)
 {
-	if (fgets(buffer, size, fileio->file) == NULL)
+	if (!fgets(buffer, size, fileio->file))
 		return ERROR_FILEIO_OPERATION_FAILED;
 
 	return ERROR_OK;