Fix an off-by-one error in LocalSampler

[krzentner]

Before, LocalSampler always gathered at least one more sample than
requested. This generally isn't a problem, but made sampling slightly
less predictable with LocalSampler.

[codecov]

Codecov Report

Merging #1156 into master will increase coverage by 4.13%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1156      +/-   ##
==========================================
+ Coverage   83.51%   87.64%   +4.13%     
==========================================
  Files         182      182              
  Lines        8639     8639              
  Branches     1102     1102              
==========================================
+ Hits         7215     7572     +357     
+ Misses       1221      866     -355     
+ Partials      203      201       -2

Impacted Files	Coverage Δ
src/garage/sampler/local_sampler.py	100% <100%> (ø)	⬆️
src/garage/sampler/stateful_pool.py	61.65% <0%> (ø)	⬆️
...rc/garage/sampler/off_policy_vectorized_sampler.py	97.75% <0%> (+1.12%)	⬆️
src/garage/tf/algos/npo.py	95.29% <0%> (+1.56%)	⬆️
src/garage/tf/misc/tensor_utils.py	76.11% <0%> (+2.98%)	⬆️
.../exploration_strategies/epsilon_greedy_strategy.py	100% <0%> (+3.7%)	⬆️
src/garage/tf/policies/categorical_cnn_policy.py	100% <0%> (+4.25%)	⬆️
src/garage/tf/baselines/continuous_mlp_baseline.py	100% <0%> (+4.54%)	⬆️
src/garage/tf/algos/vpg.py	81.81% <0%> (+9.09%)	⬆️
src/garage/tf/q_functions/base.py	100% <0%> (+13.33%)	⬆️
... and 12 more

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[krzentner]

@Mergifyio rebase

[mergify]

Command rebase: success

Branch has been successfully rebased

Hey, I reacted but my real name is @Mergifyio

[ryanjulian]

this won't merge automatically without another approving review (also it needs to pass the CI)