crypto/x509: remove redundant check for nil in tests

Comparing err variable to be not nil is redundant in this case.
The code above ensures that it is always not nil.

Updates #30208

Change-Id: I0a41601273de36a05d22270a743c0bdedeb1d0bf
GitHub-Last-Rev: 372e0fd48f90f33e266fbcdf2ccf87b9f1311c4f
GitHub-Pull-Request: golang/go#30213
Reviewed-on: https://go-review.googlesource.com/c/162439Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/crypto/x509/name_constraints_test.go

@@ -2220,10 +2220,8 @@ func TestBadNamesInSANs(t *testing.T) {
 			continue
 		}
 
-		if err != nil {
 		if str := err.Error(); !strings.Contains(str, "cannot parse ") {
 			t.Errorf("bad name %q triggered unrecognised error: %s", badName, str)
 		}
 	}
-	}
 }