opal/progress: improve performance when there are no LP callbacks

This commit adds another check to the low-priority callback
conditional that short-circuits the atomic-add if there are no
low-priority callbacks. This should improve performance in the common
case.

Signed-off-by: Nathan Hjelm <hjelmn@lanl.gov>

opal/runtime/opal_progress.c

@@ -221,7 +221,7 @@ opal_progress(void)
         events += (callbacks[i])();
     }
 
-    if ((OPAL_THREAD_ADD32((volatile int32_t *) &num_calls, 1) & 0x7) == 0) {
+    if (callbacks_lp_len > 0 && (OPAL_THREAD_ADD32((volatile int32_t *) &num_calls, 1) & 0x7) == 0) {
         /* run low priority callbacks once every 8 calls to opal_progress() */
         for (i = 0 ; i < callbacks_lp_len ; ++i) {
             events += (callbacks_lp[i])();