core: fix a potential mem leak

Found with Coverity. Fixes: CID#996438

src/core/load-fragment.c

@@ -1367,8 +1367,11 @@ int config_parse_timer(const char *unit,
         }
 
         v = new0(TimerValue, 1);
-        if (!v)
+        if (!v) {
+                if (c)
+                        free(c);
                 return log_oom();
+        }
 
         v->base = b;
         v->value = u;