Fix misc bugs in virCommandPtr

Fix misc bugs in virCommandPtr

The virCommandNewArgs() method would free the virCommandPtr
if it failed to add the args. This meant errors reported in
virCommandAddArgSet() were lost. Simply removing the check
for errors from the constructor means they can be reported
correctly later

The virCommandAddEnvPassCommon() method failed to check for
errors before reallocating the cmd->env array, causing a
potential SEGV if cmd was NULL

The virCommandAddArgSet() method needs to validate that at
least 1 element in 'val's parameter is non-NULL, otherwise
code like

    cmd = virCommandNew(binary)
    virCommandAddAtg(cmd, "foo")

Would end up trying todo  execve("foo"), if binary was
NULL.