`b_width_capped` already exists for preventing arithmetic overflow, limiting the value of `b_width` to 30. This just changes the left shifts to also use it.
The caveat of incorrect results for extremely large values of `a_width` still applies, as does the improbability of that actually happening.
This fixes#4844 (or at least, the floating point exception; the circuit still isn't valid but I think that's fine).
