util: Resolve resource leak

Need to free @groups in the parent on success similar to other
APIs (virFile*) which use virGetGroupList and virFork.

Reported by Coverity.

src/util/vircommand.c

@@ -606,6 +606,7 @@ virExec(virCommandPtr cmd)
 
         cmd->pid = pid;
 
+        VIR_FREE(groups);
         VIR_FREE(binarystr);
 
         return 0;