Fix an unguarded else clause

This fixes a gcc warning:

xicursorset.c: In function ‘main’:
xicursorset.c:77:2: warning: this ‘else’ clause does not guard... [-Wmisleading-indentation]
  else
  ^~~~
xicursorset.c:79:3: note: ...this statement, but the latter is misleadingly indented as if it is guarded by the ‘else’
   if (theme == NULL) {
   ^~