Avoid panic for empty BlockStmt

[bombsimon]

Some blocks might be empty, example
https://github.com/golang/sys/blob/master/unix/gccgo.go#L16
To avoid this from panicking we just ignore those nodes.

[coveralls]

Pull Request Test Coverage Report for Build 61

• 2 of 4 (50.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at ?%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
wsl.go	2	4	50.0%

Totals
Change from base Build 59:	0.0%
Covered Lines:
Relevant Lines:	0

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 63

• 4 of 4 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.06%) to 85.06%

---

Totals
Change from base Build 59:	0.06%
Covered Lines:	854
Relevant Lines:	1004

---

💛 - Coveralls