Clear the hypertable cache pointer on invalidation

Failure to do this results in the possible use after free if
hypertable_cache_create fails due to out of memory.

Possibly (not reproducible, since there is no OOMs anymore) observed on the running instance with:
```
[1034109]: [69d02db8.fc77d-15] [XX000] ERROR:  cache "(null)" is not initialized

[1034109]: [69d02db8.fc77d-16] 0 tsdbadmin@tsdb,app=PostgreSQL JDBC Driver [XX000] STATEMENT:  INSERT INTO sample_table (a_id, timestamp, metric, b_text, c_text, d_text, created, updated, e_bool) VALUES ($1, $2, $3, $4, $5, $6, NOW(), NOW(), false) ON CONFLICT (a_id, timestamp, metric, b_text) DO NOTHING
[891]: [69c81ac3.37b-1174] 0 @,app= [00000] LOG:  server process (PID 1034109) was terminated by signal 11: Segmentation fault
[891]: [69c81ac3.37b-1175] 0 @,app= [00000] DETAIL:  Failed process was running: INSERT INTO sample_table (a_id, timestamp, metric, b_text, c_text, d_text, created, updated, e_bool) VALUES ($1, $2, $3, $4, $5, $6, NOW(), NOW(), false) ON CONFLICT (a_id, timestamp, metric, b_text) DO NOTHING

Author: alexeyklyukin

src/hypertable_cache.c

@@ -153,6 +153,8 @@ void
 ts_hypertable_cache_invalidate_callback(void)
 {
 	ts_cache_invalidate(&hypertable_cache_current);
+	/* Prevent reusing the old pointer if cache_create aborts. */
+	hypertable_cache_current = NULL;
 	hypertable_cache_current = hypertable_cache_create();
 }