Fix inner box with unpaired split violins

[mwaskom]

Fixes #2742

With the example from that issue:

N = 30
np.random.seed(3)
data = np.random.normal(size=(N,))
df = pd.DataFrame({'Before': data,
                   'After': data + 1})
df_long = pd.melt(df, value_vars=['Before', 'After'])
ax = sns.violinplot(x='variable', y='value', data=df_long, hue='variable', split=True, inner='box', cut=1)

[codecov]

Codecov Report

Merging #2814 (d4fceee) into master (34b8873) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2814   +/-   ##
=======================================
  Coverage   97.96%   97.96%           
=======================================
  Files          67       67           
  Lines       21756    21761    +5     
=======================================
+ Hits        21313    21318    +5     
  Misses        443      443           

Impacted Files	Coverage Δ
seaborn/categorical.py	98.24% <100.00%> (ø)
seaborn/tests/test_categorical.py	99.76% <100.00%> (+<0.01%)	⬆️

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