Check that info.NCPU is not zero (#29302) (#29889)

## What does this PR do?

This adds a check for 0 when determining the concurrency limit used by mage during a build.

## Why is it important?

`docker info -f '{{ json .}}'` exits with 0 even if there was an error, so the check for `err == nil` doesn't catch errors, and the default int value of 0 is used when trying to read `info.NCPU`. This leads to the confusing behaviour where the build stops and hangs forever with no errors and no indication that anything has gone wrong unless you build with --verbose.

(cherry picked from commit 10f850e)

Co-authored-by: Maxwell Borden <[email protected]>

dev-tools/mage/common.go

@@ -528,7 +528,9 @@ func numParallel() int {
 	maxParallel := runtime.NumCPU()
 
 	info, err := GetDockerInfo()
-	if err == nil && info.NCPU < maxParallel {
+	// Check that info.NCPU != 0 since docker info doesn't return with an
+	// error status if communcation with the daemon failed.
+	if err == nil && info.NCPU != 0 && info.NCPU < maxParallel {
 		maxParallel = info.NCPU
 	}