Fix #253: Acquire sufficient bytes for string encoding buffers

[lujjjh]

What this PR does:

Acquire sufficient bytes for string encoding buffers.

Which issue(s) this PR fixes:

Fixes #253

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

- fix index out of range in some string encoding cases

[codecov-io]

Codecov Report

Merging #255 (0b4c6a8) into master (ab7254f) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #255   +/-   ##
=======================================
  Coverage   67.61%   67.61%           
=======================================
  Files          25       25           
  Lines        2671     2671           
=======================================
  Hits         1806     1806           
  Misses        644      644           
  Partials      221      221           

Impacted Files	Coverage Δ
string.go	70.70% <100.00%> (ø)

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