removes annoying warning

```
[ 31%] Building CXX object numgrid/CMakeFiles/numgrid-objects.dir/lebedev/sphere_lebedev_rule.cpp.o
/home/scemama/Downloads/numgrid/numgrid/lebedev/sphere_lebedev_rule.cpp: In function ‘void timestamp()’:
/home/scemama/Downloads/numgrid/numgrid/lebedev/sphere_lebedev_rule.cpp:6483:10: warning: variable ‘len’ set but not used [-Wunused-but-set-variable]
   size_t len;
          ^~~
```

Author: scemama

numgrid/lebedev/sphere_lebedev_rule.cpp

@@ -6487,6 +6487,14 @@ void timestamp ( void )
   tm = localtime ( &now );
 
   len = strftime ( time_buffer, TIME_SIZE, "%d %B %Y %I:%M:%S %p", tm );
+  if (len == 0)
+  {
+    fprintf ( stderr, "\n" );
+    fprintf ( stderr, "TIMESTAMP - Fatal error!\n" );
+    fprintf ( stderr, "  TIME_SIZE constant too small.\n" );
+    exit ( 1 );
+  }
+ 
 
   fprintf ( stdout, "%s\n", time_buffer );