rsrv: reject a non-finite beacon period

The beacon-period gate in online_notify already rejected a parse
failure and a non-positive value, but epicsScanDouble parses "nan" and
"inf", and maxPeriod<=0.0 is false for both, so a non-finite period
reached the beacon delay loop. A NaN maxdelay makes delay<maxdelay
false forever, pinning the beacon interval at the 0.02 s startup value
- a ~50 Hz beacon flood on every configured interface; an infinite
maxdelay lets the doubling delay grow without bound, silencing beacons
entirely.

Gate on finite() (from epicsMath.h), matching the client-side
EPICS_CA_MAX_SEARCH_PERIOD handling in udpiiu.cpp, and name the actual
problem in the diagnostic instead of "float fetch failed" (the message
already covered the non-positive case, which is not a fetch failure).

modules/database/src/ioc/rsrv/online_notify.c

@@ -27,6 +27,7 @@
 #include "addrList.h"
 #include "dbDefs.h"
 #include "envDefs.h"
+#include "epicsMath.h"
 #include "errlog.h"
 #include "osiSock.h"
 #include "taskwd.h"
@@ -55,9 +56,9 @@ void rsrv_online_notify_task(void *pParm)
     else {
         longStatus = envGetDoubleConfigParam ( & EPICS_CA_BEACON_PERIOD, & maxPeriod );
     }
-    if (longStatus || maxPeriod<=0.0) {
+    if (longStatus || !finite(maxPeriod) || maxPeriod<=0.0) {
         maxPeriod = 15.0;
-        epicsPrintf ("EPICS \"%s\" float fetch failed\n",
+        epicsPrintf ("EPICS \"%s\" was not a positive real number\n",
                         EPICS_CAS_BEACON_PERIOD.name);
         epicsPrintf ("Setting \"%s\" = %f\n",
             EPICS_CAS_BEACON_PERIOD.name, maxPeriod);