parser: fix string extra check

[visualfc]

fix parser panic

run "

panic: runtime error: slice bounds out of range [1:0] [recovered]
	panic: runtime error: slice bounds out of range [1:0]
goroutine 1 [running]:
github.com/goplus/gop/parser.parseFile.func1()
	/Users/vfc/goplus/gop/parser/interface.go:96 +0x1f2
panic({0x1680080?, 0xc0000c81c8?})

fixed to

bug.gop:1:5: string literal not terminated

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.23%. Comparing base (ffe6653) to head (45c69bf).
Report is 20 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1782      +/-   ##
==========================================
+ Coverage   92.22%   92.23%   +0.01%     
==========================================
  Files          40       40              
  Lines       11021    11023       +2     
==========================================
+ Hits        10164    10167       +3     
  Misses        694      694              
+ Partials      163      162       -1     

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