SetLimit crashes with arm32

[jduranf]

Using arm32, when the function buffer.SetLimit(bl) is called in source_node.go, the service crash:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x11f08]

goroutine 20 [running]:
runtime/internal/atomic.goStore64(0x25ed134, 0x19000, 0x0)
	/usr/local/go/src/runtime/internal/atomic/atomic_arm.go:144 +0x1c
github.com/emqx/kuiper/xstream/nodes.(*DynamicChannelBuffer).SetLimit(...)
	/home/jduran/go/src/github.com/emqx/kuiper/xstream/nodes/dynamic_channel_buffer.go:29
github.com/emqx/kuiper/xstream/nodes.(*SourceNode).Open.func1.1(0x25bc140, 0x25b6240, 0x70e270, 0x25f3bc0, 0x70d918, 0x25f3900, 0x25ed000, 0x25ef76c, 0x0)
	/home/jduran/go/src/github.com/emqx/kuiper/xstream/nodes/source_node.go:127 +0xca0
created by github.com/emqx/kuiper/xstream/nodes.(*SourceNode).Open.func1
	/home/jduran/go/src/github.com/emqx/kuiper/xstream/nodes/source_node.go:80 +0x1dc

This is due to a bad alignment:
golang/go#23345
https://stackoverflow.com/questions/28670232/atomic-addint64-causes-invalid-memory-address-or-nil-pointer-dereference

And can be easily fixed by changing the variables order in this structure:

type DynamicChannelBuffer struct {
	In     chan api.SourceTuple
	Out    chan api.SourceTuple
	buffer []api.SourceTuple
	limit  int64
}

by:

type DynamicChannelBuffer struct {
	limit  int64
	In     chan api.SourceTuple
	Out    chan api.SourceTuple
	buffer []api.SourceTuple
}

[ngjaying]

Thank you @jduranf for the very detail explanation. PR merged.

[jduranf]

Thank you for the quick fix!