matrix-reader: Fix possibly-zero second argument to xcalloc().

matrix-reader: Fix possibly-zero second argument to xcalloc().

xcalloc() is documented to disallow zero for its second argument:

  /* Allocate zeroed memory for N elements of S bytes, with error
     checking.  S must be nonzero.  */
  void *
  xcalloc (size_t n, size_t s)

This code didn't always call it properly.

Found with AFL++.
Thanks to Andrea Fioraldi for reporting the problem.
Bug #58600.