AP_Terrain: leave ENABLE as 1 if memory alloc fails

libraries/AP_Terrain/AP_Terrain.cpp

@@ -376,7 +376,7 @@ void AP_Terrain::log_terrain_data()
  */
 bool AP_Terrain::allocate(void)
 {
-    if (enable == 0) {
+    if (enable == 0 || memory_alloc_failed) {
         return false;
     }
     if (cache != nullptr) {
@@ -384,13 +384,11 @@ bool AP_Terrain::allocate(void)
     }
     cache = (struct grid_cache *)calloc(TERRAIN_GRID_BLOCK_CACHE_SIZE, sizeof(cache[0]));
     if (cache == nullptr) {
-        enable.set(0);
         gcs().send_text(MAV_SEVERITY_CRITICAL, "Terrain: Allocation failed");
         memory_alloc_failed = true;
         return false;
     }
     cache_size = TERRAIN_GRID_BLOCK_CACHE_SIZE;
-    memory_alloc_failed = false;
     return true;
 }