safe_read_from: bump default size limit to 1 MiB to match pmxcfs

Done in a similar spirit as commit 8fb28ab914

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>

src/PVE/Tools.pm

@@ -302,7 +302,7 @@ sub safe_read_from {
     my ($fh, $max, $oneline, $filename) = @_;
 
     # pmxcfs file size limit
-    $max = 512*1024 if !$max;
+    $max = 1024 * 1024 if !$max;
 
     my $subject = defined($filename) ? "file '$filename'" : 'input';