buffer: improve copy() performance

[seishun]

There is no need to create a slice when sourceEnd is out of bounds.

Results of included benchmark:

                                                                           confidence improvement accuracy (*)     (**)    (***)
 buffers\\buffer-copy.js n=6000000 oob='false' partial='false' bytes=0                     3.30 %      ±4.96%  ±6.62%  ±8.65%
 buffers\\buffer-copy.js n=6000000 oob='false' partial='false' bytes=128                  -0.66 %      ±3.54%  ±4.71%  ±6.13%
 buffers\\buffer-copy.js n=6000000 oob='false' partial='false' bytes=32768                -3.02 %      ±8.44% ±11.30% ±14.83%
 buffers\\buffer-copy.js n=6000000 oob='false' partial='false' bytes=8                     2.80 %      ±5.54%  ±7.37%  ±9.61%
 buffers\\buffer-copy.js n=6000000 oob='false' partial='true' bytes=0                      2.29 %      ±4.58%  ±6.10%  ±7.98%
 buffers\\buffer-copy.js n=6000000 oob='false' partial='true' bytes=128                    1.90 %      ±3.39%  ±4.52%  ±5.88%
 buffers\\buffer-copy.js n=6000000 oob='false' partial='true' bytes=32768                  0.23 %      ±7.59% ±10.10% ±13.16%
 buffers\\buffer-copy.js n=6000000 oob='false' partial='true' bytes=8                      2.75 %      ±4.06%  ±5.42%  ±7.07%
 buffers\\buffer-copy.js n=6000000 oob='true' partial='false' bytes=0                      2.68 %      ±4.89%  ±6.52%  ±8.50%
 buffers\\buffer-copy.js n=6000000 oob='true' partial='false' bytes=128           ***    135.43 %      ±7.70% ±10.31% ±13.57%
 buffers\\buffer-copy.js n=6000000 oob='true' partial='false' bytes=32768           *      5.98 %      ±4.85%  ±6.46%  ±8.40%
 buffers\\buffer-copy.js n=6000000 oob='true' partial='false' bytes=8             ***    142.55 %     ±10.22% ±13.71% ±18.06%
 buffers\\buffer-copy.js n=6000000 oob='true' partial='true' bytes=0                       4.42 %      ±4.67%  ±6.26%  ±8.23%
 buffers\\buffer-copy.js n=6000000 oob='true' partial='true' bytes=128                     0.66 %      ±2.75%  ±3.66%  ±4.77%
 buffers\\buffer-copy.js n=6000000 oob='true' partial='true' bytes=32768                  -1.10 %      ±7.26%  ±9.67% ±12.59%
 buffers\\buffer-copy.js n=6000000 oob='true' partial='true' bytes=8                       2.74 %      ±3.92%  ±5.23%  ±6.82%

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/31218/

[addaleax]

Landed in 339690c