To improve function's defensive programming

[tom-gu]

Especially:

• https://github.com/bd-R/bdchecks/blob/dev/R/filter.R
• https://github.com/bd-R/bdchecks/blob/dev/R/summary.R

Here is a good example of why it's so important: bd-R/bdchecks.app#37 (comment)