extstore: fail to start if given no disk space

should avoid accidentally starting up with extstore enabled but no atual
disk space assigned.

storage.c

@@ -1083,6 +1083,10 @@ struct extstore_conf_file *storage_conf_parse(char *arg, unsigned int page_size)
     // page_count is nearest-but-not-larger-than pages * psize
     cf->page_count = multiplier / page_size;
     assert(page_size * cf->page_count <= multiplier);
+    if (cf->page_count == 0) {
+        fprintf(stderr, "supplied ext_path has zero size, cannot use\n");
+        goto error;
+    }
 
     // final token would be a default free bucket
     p = strtok_r(NULL, ",", &b);

t/extstore.t

@@ -17,6 +17,16 @@ if (!supports_extstore()) {
 
 $ext_path = "/tmp/extstore.$$";
 
+eval {
+    my $server = new_memcached("-o ext_path=$ext_path:0m");
+};
+ok($@, "failed to start server with zero pages assigned");
+
+eval {
+    my $server = new_memcached("-o ext_path=$ext_path:1GB");
+};
+ok($@, "failed to start server with invalid path size");
+
 my $server = new_memcached("-m 64 -U 0 -o ext_page_size=8,ext_wbuf_size=2,ext_threads=1,ext_io_depth=2,ext_item_size=512,ext_item_age=2,ext_recache_rate=10000,ext_max_frag=0.9,ext_path=$ext_path:64m,slab_automove=0,ext_compact_under=1,ext_max_sleep=100000");
 my $sock = $server->sock;