Fix infinite loop when a file ends with a short identifier

[alangpierce]

We weren't properly checking for end-of-file when going through the readWord
tree.

[codecov-io]

Codecov Report

Merging #363 into master will increase coverage by 0.05%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #363      +/-   ##
=========================================
+ Coverage   79.24%   79.3%   +0.05%     
=========================================
  Files          44      44              
  Lines        4847    5068     +221     
  Branches     1133    1133              
=========================================
+ Hits         3841    4019     +178     
- Misses        706     747      +41     
- Partials      300     302       +2

Impacted Files	Coverage Δ
src/parser/tokenizer/readWord.ts	85.36% <100%> (+3.54%)	⬆️
src/util/formatTokens.ts	70.58% <0%> (-10.06%)	⬇️
src/parser/util/identifier.ts	90.9% <0%> (-4.1%)	⬇️
src/parser/plugins/jsx/index.ts	87.33% <0%> (-1.88%)	⬇️
src/parser/plugins/flow.ts	62.14% <0%> (-1.59%)	⬇️
src/util/getClassInfo.ts	85.12% <0%> (-1.21%)	⬇️
src/parser/traverser/statement.ts	80.14% <0%> (-1.17%)	⬇️
src/util/isIdentifier.ts	100% <0%> (ø)	⬆️
src/parser/util/whitespace.ts	100% <0%> (ø)	⬆️
src/parser/traverser/index.ts	100% <0%> (ø)	⬆️
... and 14 more

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