go/cmd, crypto/x509, net/textproto, html/template: fix minor issues with nil values

Remove redundant checks for nil value, add missing nil checks to prevent tests from failing with 'nil pointer dereference'.

Fixes #30208.

Change-Id: I59091ba4014afcb5300567fd7e73fea43c6bb2ee
GitHub-Last-Rev: 20501470bbb8ad8fd60f6f87b4594ab64fcfdff1
GitHub-Pull-Request: golang/go#30226
Reviewed-on: https://go-review.googlesource.com/c/go/+/162657Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/internal/work/exec.go

@@ -104,9 +104,7 @@ func (b *Builder) Do(root *Action) {
 		var err error
 
 		if a.Func != nil && (!a.Failed || a.IgnoreFail) {
-			if err == nil {
-				err = a.Func(b, a)
-			}
+			err = a.Func(b, a)
 		}
 
 		// The actions run in parallel but all the updates to the