fix gctx init slice bounds out of range on ios platform (#2062)

os/gctx/gctx.go

@@ -33,6 +33,9 @@ func init() {
 	i := 0
 	for _, s := range os.Environ() {
 		i = strings.IndexByte(s, '=')
+		if i == -1 {
+			continue
+		}
 		m[s[0:i]] = s[i+1:]
 	}
 	// OpenTelemetry from environments.