Fixed missing argument
Properly checking bodies during update step

When calculating forces acting on a body I omitted a check for body
identity. This resulted in a rare case of 0/0 operation, which caused
NaN, and which in turn resutled in v8's deoptimization.

This fix boosts performance from 4.43 ops/sec ±1.46% to 8.87 ops/sec ±2.88%
0.0.2