kernel/fs: On write, relax address check on 0 length and NULL ptr

* IEEE Std 1003.1-2017, read, write
* golang performs a write of 0 length with a NULL buffer to
  "create" an empty file. We return BAD_ADDRESS for this.
* If condition above occurs, continue as a length of 0 means
  we won't read the passed buffer.

Change-Id: I8718abb92f5865a7b6f4fb7f2b74f803497ebef0
Reviewed-on: https://review.haiku-os.org/c/haiku/+/3285
Reviewed-by: Alex von Gluck IV <[email protected]>
Reviewed-by: Adrien Destugues <[email protected]>

src/system/kernel/fs/fd.cpp

@@ -731,9 +731,6 @@ common_close(int fd, bool kernel)
 static ssize_t
 common_user_io(int fd, off_t pos, void* buffer, size_t length, bool write)
 {
-	if (!IS_USER_ADDRESS(buffer))
-		return B_BAD_ADDRESS;
-
 	if (pos < -1)
 		return B_BAD_VALUE;
 
@@ -758,6 +755,12 @@ common_user_io(int fd, off_t pos, void* buffer, size_t length, bool write)
 		return B_BAD_VALUE;
 	}
 
+	if (length == 0)
+		return 0;
+
+	if (!IS_USER_ADDRESS(buffer))
+		return B_BAD_ADDRESS;
+
 	SyscallRestartWrapper<status_t> status;
 
 	if (write)