[fx][set] Avoid nil pointer err on file not found

Before this fix, `fx set` would NPE if user passes in a
`--fint-params-path` pointing to a non-existent file. With this fix it
correctly errors out with `no such file or directory`.

Change-Id: I4452c9fd0bf4d0f532eb8bd2340971809e6b0439
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1086372
Reviewed-by: Oliver Newman <[email protected]>
Commit-Queue: Auto-Submit <[email protected]>
Fuchsia-Auto-Submit: Jay Zhuang <[email protected]>

Author: jayzhuang

tools/build/fx-set/cmd/main.go

@@ -126,10 +126,10 @@ func mainImpl(ctx context.Context) error {
 			path = filepath.Join(args.checkoutDir, path)
 		}
 		staticSpec, err = fint.ReadStatic(path)
-		staticSpec.GnArgs = append(staticSpec.GnArgs, args.gnArgs...)
 		if err != nil {
 			return err
 		}
+		staticSpec.GnArgs = append(staticSpec.GnArgs, args.gnArgs...)
 	}
 
 	contextSpec := &fintpb.Context{