avutil/md5: move loop inside inner function

AMD Athlon(tm) II X3 450 Processor

size: 1048576  runs: 1024  time:    5.660 +- 0.023
size: 1048576  runs: 1024  time:    5.661 +- 0.030
size: 1048576  runs: 1024  time:    5.656 +- 0.022
size: 1048576  runs: 1024  time:    5.647 +- 0.026

size: 1048576  runs: 1024  time:    5.428 +- 0.037
size: 1048576  runs: 1024  time:    5.426 +- 0.034
size: 1048576  runs: 1024  time:    5.426 +- 0.034
size: 1048576  runs: 1024  time:    5.428 +- 0.038

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>