azureblob: fix incorrect size after --azureblob-no-head-object patch

In

05f128868f47b1b5 azureblob: add --azureblob-no-head-object

we incorrectly parsed the size of the object as the Content-Length of
the returned header. This is incorrect in the presense of Range
requests.

This fixes the problem by parsing the Content-Range header if
avaialble to read the correct length from if a Range request was
issued.

See: #5734

backend/azureblob/azureblob.go

@@ -16,6 +16,7 @@ import (
 	"net/http"
 	"net/url"
 	"path"
+	"strconv"
 	"strings"
 	"sync"
 	"time"
@@ -1388,6 +1389,21 @@ func (o *Object) decodeMetaDataFromDownloadResponse(info *azblob.DownloadRespons
 	o.accessTier = o.AccessTier()
 	o.setMetadata(metadata)
 
+	// If it was a Range request, the size is wrong, so correct it
+	if contentRange := info.ContentRange(); contentRange != "" {
+		slash := strings.IndexRune(contentRange, '/')
+		if slash >= 0 {
+			i, err := strconv.ParseInt(contentRange[slash+1:], 10, 64)
+			if err == nil {
+				o.size = i
+			} else {
+				fs.Debugf(o, "Failed to find parse integer from in %q: %v", contentRange, err)
+			}
+		} else {
+			fs.Debugf(o, "Failed to find length in %q", contentRange)
+		}
+	}
+
 	return nil
 }