HADOOP-19011. Possible ConcurrentModificationException if Exec command fails (#6358)

(cherry picked from commit cf21f35526fca2d6c2c0518ba0a5d0dc2057f8ad)

hadoop-maven-plugins/src/main/java/org/apache/hadoop/maven/plugin/util/Exec.java

@@ -72,14 +72,14 @@ public class Exec {
       stdOut.start();
       stdErr.start();
       retCode = p.waitFor();
+      stdOut.join();
+      stdErr.join();
       if (retCode != 0) {
         mojo.getLog().warn(command + " failed with error code " + retCode);
         for (String s : stdErr.getOutput()) {
           mojo.getLog().debug(s);
         }
       }
-      stdOut.join();
-      stdErr.join();
       output.addAll(stdOut.getOutput());
       if (errors != null) {
         errors.addAll(stdErr.getOutput());