Avoid object names with '//' to avoid hash inconsistencies (#8946)

This is to fix a situation where an object name incorrectly
is sent with '//' in its path heirarchy, we should reject
such object names because they may be hashed to a set where
the object might not originally belong because, this can
cause situations where once object is uploaded we cannot
delete it anymore.

Fixes #8873

cmd/object-api-utils_test.go

@@ -122,7 +122,9 @@ func TestIsValidObjectName(t *testing.T) {
 		{" ../etc", false},
 		{"./././", false},
 		{"./etc", false},
-		{"contains-\\-backslash", false},
+		{`contains-\-backslash`, false},
+		{`contains//double/forwardslash`, false},
+		{`//contains/double-forwardslash-prefix`, false},
 		{string([]byte{0xff, 0xfe, 0xfd}), false},
 	}