zstd: Tweak DecodeAll standard allocs (#295)

Default to compressed size * 2 if no frame size and no destination buffer given.

zstd/decoder.go

@@ -323,19 +323,23 @@ func (d *Decoder) DecodeAll(input, dst []byte) ([]byte, error) {
 		}
 		if frame.FrameContentSize > 0 && frame.FrameContentSize < 1<<30 {
 			// Never preallocate moe than 1 GB up front.
-			if uint64(cap(dst)) < frame.FrameContentSize {
+			if cap(dst)-len(dst) < int(frame.FrameContentSize) {
 				dst2 := make([]byte, len(dst), len(dst)+int(frame.FrameContentSize))
 				copy(dst2, dst)
 				dst = dst2
 			}
 		}
 		if cap(dst) == 0 {
-			// Allocate window size * 2 by default if nothing is provided and we didn't get frame content size.
-			size := frame.WindowSize * 2
+			// Allocate len(input) * 2 by default if nothing is provided
+			// and we didn't get frame content size.
+			size := len(input) * 2
 			// Cap to 1 MB.
 			if size > 1<<20 {
 				size = 1 << 20
 			}
+			if uint64(size) > d.o.maxDecodedSize {
+				size = int(d.o.maxDecodedSize)
+			}
 			dst = make([]byte, 0, size)
 		}