Place bounds check within memUvarintReader's ReadUvarint

This bounds check is to return an empty uint64 while
decoding a 64-bit integer, should the reader have already
read all the bytes available.

For: blevesearch/bleve#1651

```
Some context on the memUvarintReader ..

The code here reflects ReadUVarint(..) from
https://cs.opensource.google/go/go/+/refs/tags/go1.13.7:src/encoding/binary/varint.go

But as reported here: golang/go#40618,
it appears it contained an error prone path that was addressed in 1.15.
```

memuvarint.go

@@ -44,6 +44,11 @@ func (r *memUvarintReader) ReadUvarint() (uint64, error) {
 	var C = r.C
 	var S = r.S
 
+	if C >= len(S) {
+		// nothing else to read
+		return 0, nil
+	}
+
 	for {
 		b := S[C]
 		C++
@@ -59,7 +64,7 @@ func (r *memUvarintReader) ReadUvarint() (uint64, error) {
 			// why the "extra" >= check?  The normal case is that s <
 			// 63, so we check this single >= guard first so that we
 			// hit the normal, nil-error return pathway sooner.
-			if s >= 63 && (s > 63 || s == 63 && b > 1) {
+			if s >= 63 && (s > 63 || b > 1) {
 				return 0, fmt.Errorf("memUvarintReader overflow")
 			}
 
@@ -74,6 +79,10 @@ func (r *memUvarintReader) ReadUvarint() (uint64, error) {
 // SkipUvarint skips ahead one encoded uint64.
 func (r *memUvarintReader) SkipUvarint() {
 	for {
+		if r.C >= len(r.S) {
+			return
+		}
+
 		b := r.S[r.C]
 		r.C++