Put an upper bound on test_workqueue -I argument

This makes coverity happy, since it doesn't like us looping against
a value which might be INT_MAX.

This is CID 1268069

src/test/test_workqueue.c

@@ -18,6 +18,8 @@
 #include <event.h>
 #endif
 
+#define MAX_INFLIGHT (1<<16)
+
 static int opt_verbose = 0;
 static int opt_n_threads = 8;
 static int opt_n_items = 10000;
@@ -348,7 +350,7 @@ main(int argc, char **argv)
   }
   if (opt_n_threads < 1 ||
       opt_n_items < 1 || opt_n_inflight < 1 || opt_n_lowwater < 0 ||
-      opt_n_cancel > opt_n_inflight ||
+      opt_n_cancel > opt_n_inflight || opt_n_inflight > MAX_INFLIGHT ||
       opt_ratio_rsa < 0) {
     help();
     return 1;