rts: Initialize card table in newArray#
In #19143 (closed) I noticed that newArray# failed to initialize the card table of newly-allocated arrays. However, embarrassingly, I then only fixed the issue in newArrayArray# and, in so doing, introduced the potential for an integer underflow on zero-length arrays (#21962 (closed)).
Here I fix the issue in newArray#, this time ensuring that we do not underflow in pathological cases.
Fixes #19143 (closed).