Fix two crashers found by fuzzing

https://github.com/dvyukov/go-fuzz

Author: dgryski

reader.go

@@ -141,7 +141,7 @@ func Decode(dst, src []byte) ([]byte, error) {
 length += ln
 }
 
-if int(d.spos+length) > len(d.src) {
+if int(d.spos+length) > len(d.src) || int(d.dpos+length) > len(d.dst) {
 return nil, ErrCorrupt
 }
 
@@ -179,7 +179,12 @@ func Decode(dst, src []byte) ([]byte, error) {
 }
 
 literal := d.dpos - d.ref
+
 if literal < 4 {
+if int(d.dpos+4) > len(d.dst) {
+return nil, ErrCorrupt
+}
+
 d.cp(4, decr[literal])
 } else {
 length += 4