Issue #3655: Fix NPE in NeedBraces on single line default stmt

[liscju]

Problem in #3655 was that isSingleLineDefault assumed that default stmt
has slist. This commit fixes this by checking if slist exists.

[romani]

Looks like git commit message is multi-line, it should be single line.
Please update PR description to reference issue number.

[codecov-io]

Current coverage is 100% (diff: 100%)

Merging #3673 into master will not change coverage

@@           master   #3673   diff @@
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  Files         275     275          
  Lines       13571   13573     +2   
  Methods         0       0          
  Messages        0       0          
  Branches     3051    3052     +1   
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+ Hits        13571   13573     +2   
  Misses          0       0          
  Partials        0       0          

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[romani]

good, thanks a lot.

please do the same fix for "isSingleLineCase" it is not referenced in issue test case, but problem is the same.

2. please launch regression testing for this option "allowSingleLineStatement" for all tokens.
   by https://github.com/checkstyle/contribution/tree/master/checkstyle-tester on all project at https://github.com/checkstyle/contribution/blob/master/checkstyle-tester/projects-to-test-on.properties
   just to make sure we did not left any similar Exceptional case.
   Code change is simple enough, I do not how can we change validation results, so tst for Exceptions only is enough.

[romani]

please do the same fix for "isSingleLineCase" it is not referenced in issue test case, but problem is the same.

It is possible:

 |--CASE_GROUP -> CASE_GROUP [9:12]
 |   `--LITERAL_CASE -> case [9:12]
 |       |--EXPR -> EXPR [9:17]
 |       |   `--NUM_INT -> 1 [9:17]
 |       `--COLON -> : [9:18]
 `--RCURLY -> } [10:8]

[romani]

one more exception on case:

@interface Example {
    String priority() default "value";
}

exception:

java.lang.NullPointerException
	at com.puppycrawl.tools.checkstyle.checks.blocks.NeedBracesCheck.isSingleLineDefault(NeedBracesCheck.java:446)
	at com.puppycrawl.tools.checkstyle.checks.blocks.NeedBracesCheck.isSingleLineStatement(NeedBracesCheck.java:289)
	at com.puppycrawl.tools.checkstyle.checks.blocks.NeedBracesCheck.isSkipStatement(NeedBracesCheck.java:222)
	at com.puppycrawl.tools.checkstyle.checks.blocks.NeedBracesCheck.visitToken(NeedBracesCheck.java:208)
	at com.puppycrawl.tools.checkstyle.TreeWalker.notifyVisit(TreeWalker.java:361)
	at com.puppycrawl.tools.checkstyle.TreeWalker.processIter(TreeWalker.java:498)
	at com.puppycrawl.tools.checkstyle.TreeWalker.walk(TreeWalker.java:303)
	at com.puppycrawl.tools.checkstyle.TreeWalker.processFiltered(TreeWalker.java:178)
	at com.puppycrawl.tools.checkstyle.api.AbstractFileSetCheck.process(AbstractFileSetCheck.java:78)
	at com.puppycrawl.tools.checkstyle.Checker.processFile(Checker.java:314)
	at com.puppycrawl.tools.checkstyle.Checker.processFiles(Checker.java:284)
	at com.puppycrawl.tools.checkstyle.Checker.process(Checker.java:211)
	at com.puppycrawl.tools.checkstyle.Main.runCheckstyle(Main.java:425)
	at com.puppycrawl.tools.checkstyle.Main.runCli(Main.java:359)
	at com.puppycrawl.tools.checkstyle.Main.main(Main.java:174)

ast:

ANNOTATION_DEF -> ANNOTATION_DEF [11:0]
|--MODIFIERS -> MODIFIERS [11:0]
|--AT -> @ [11:0]
|--LITERAL_INTERFACE -> interface [11:1]
|--IDENT -> Example [11:11]
`--OBJBLOCK -> OBJBLOCK [11:19]
    |--LCURLY -> { [11:19]
    |--ANNOTATION_FIELD_DEF -> ANNOTATION_FIELD_DEF [12:4]
    |   |--MODIFIERS -> MODIFIERS [12:4]
    |   |--TYPE -> TYPE [12:4]
    |   |   `--IDENT -> String [12:4]
    |   |--IDENT -> priority [12:11]
    |   |--LPAREN -> ( [12:19]
    |   |--RPAREN -> ) [12:20]
    |   |--LITERAL_DEFAULT -> default [12:22]
    |   |   `--EXPR -> EXPR [12:30]
    |   |       `--STRING_LITERAL -> "value" [12:30]
    |   `--SEMI -> ; [12:37]
    `--RCURLY -> } [13:0]

[romani]

commit was updated and merged in scope of #3682 .

Stress testing was done on all projects in checkstyle-tester