AP_NavEKF2: clean up init failure handling

only attempt to allocate memory once

libraries/AP_NavEKF2/AP_NavEKF2.cpp

@@ -621,6 +621,11 @@ void NavEKF2::check_log_write(void)
 // Initialise the filter
 bool NavEKF2::InitialiseFilter(void)
 {
+    // Return immediately if there is insufficient memory
+    if (core_malloc_failed) {
+        return false;
+    }
+
     initFailure = InitFailures::UNKNOWN;
     if (_enable == 0) {
         if (_ahrs->get_ekf_type() == 2) {
@@ -666,16 +671,16 @@ bool NavEKF2::InitialiseFilter(void)
         // check if there is enough memory to create the EKF cores
         if (hal.util->available_memory() < sizeof(NavEKF2_core)*num_cores + 4096) {
             initFailure = InitFailures::NO_MEM;
+            core_malloc_failed = true;
             gcs().send_text(MAV_SEVERITY_CRITICAL, "NavEKF2: not enough memory available");
-            _enable.set(0);
             return false;
         }
 
         // try to allocate from CCM RAM, fallback to Normal RAM if not available or full
         core = (NavEKF2_core*)hal.util->malloc_type(sizeof(NavEKF2_core)*num_cores, AP_HAL::Util::MEM_FAST);
         if (core == nullptr) {
-            _enable.set(0);
             initFailure = InitFailures::NO_MEM;
+            core_malloc_failed = true;
             gcs().send_text(MAV_SEVERITY_CRITICAL, "NavEKF2: memory allocation failed");
             return false;
         }

libraries/AP_NavEKF2/AP_NavEKF2.h

@@ -359,6 +359,7 @@ private:
     uint8_t num_cores; // number of allocated cores
     uint8_t primary;   // current primary core
     NavEKF2_core *core = nullptr;
+    bool core_malloc_failed;
     const AP_AHRS *_ahrs;
     const RangeFinder &_rng;