Fix memory leak in emitter

[jpflorijn]

For every map, sequence and scalar emitted, lyaml leaks six bytes, because the mapping style parameter is strdup()-ed but not freed.

This issue was first reported in #35.

The test code included in that report consistently showed leaks under at least lua5.1 and luajit 2.1.0-beta3; the following version under tighter GC settings makes the issue more visible though:

collectgarbage("setpause", 103)
collectgarbage("setstepmul", 1600)
local yaml=require("lyaml")
print(collectgarbage("count"),"kb")
for i = 1, 10 do
    for j=1,1000 do
        yaml.dump({a="b", c = {1, 2, 3}, d = {{3, 2, 1}, true, false}})
    end
    print(collectgarbage("count"),"kb")
    collectgarbage()
    print(collectgarbage("count"),"kb")
end

Fixed by freeing style analogously to encoding and type.

[codecov[bot]]

Codecov Report

Base: 97.14% // Head: 97.14% // No change to project coverage :thumbsup:

Coverage data is based on head (5c7ab75) compared to base (0701528). Patch has no changes to coverable lines.

Additional details and impacted files

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[gvvaughan]

Thank you so much for making the time to debug this and submit a PR for the fix! Merged and released as v6.2.8.