OpenAPI: inspect token unless the type_stack is empty

[masatake]

A ctags process crashes with following input (input.yml):

{x:

@segoon, could you look at this one?
This should be merged before #3258.

[codecov]

Codecov Report

Merging #3261 (1e85d24) into master (b0bb484) will decrease coverage by 0.09%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3261      +/-   ##
==========================================
- Coverage   85.30%   85.20%   -0.10%     
==========================================
  Files         208      210       +2     
  Lines       49507    49682     +175     
==========================================
+ Hits        42230    42330     +100     
- Misses       7277     7352      +75     

Impacted Files	Coverage Δ
parsers/openapi.c	95.89% <100.00%> (ø)
dsl/dsl.c	80.33% <0.00%> (-1.34%)	⬇️
dsl/sorter.c	64.33% <0.00%> (-0.92%)	⬇️
main/field.c	92.73% <0.00%> (ø)
main/vstring.h	100.00% <0.00%> (ø)
main/routines.c	81.41% <0.00%> (ø)
main/selectors.c	79.79% <0.00%> (ø)
dsl/formatter.c	70.31% <0.00%> (ø)
extra-cmds/readtags-stub.c	0.00% <0.00%> (ø)
... and 2 more

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