Calc: fix interval entry (bug#39040)

* lisp/calc/calc.el (calcDigit-start): Initialise calc-prev-char to
something more reasonable, so that non-algebraic entry of intervals
whose start is a single digit, like (1..10), works properly.
Reported by Michel Schinz.
1 job for emacs-27 in 57 minutes and 51 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #4730
test-all

00:57:51