powerman: correct potential segfault

Problem: In _process_setresult() if the plug name is not found
in the inputted arglist, it can result in a segfault.  This
can occur when power operations on dependent targets (e.g. the
parent of node needs to be powered on) have errors, leading to
an unexpected host having a "power result".

Check that arg is non-NULL before trying to dereference it.

Fixes #197

src/powerman/device.c

@@ -1178,7 +1178,7 @@ static bool _process_setresult(Device *dev, Action *act, ExecCtx *e)
                 arg->val = xstrdup(str);
             }
 
-            if (result != RT_SUCCESS) {
+            if (arg && result != RT_SUCCESS) {
                 char strbuf[1024];
                 snprintf(strbuf, sizeof(strbuf), "%s", arg->val);
                 /* remove trailing carriage return or newline */