Enclose dangling else in braces and fix -Wmisleading-indentation

[nobu]

date_strptime.c:253:324: warning: misleading indentation;
      statement is not part of the previous 'if' [-Wmisleading-indentation]
  253 |   ...((VALUE)RUBY_Qtrue)); return 0; } while (0); si += l; } while (0);
      |                                                   ^
date_strptime.c:252:7: note: previous statement is here
  252 |       else
      |       ^