fix buffer overrun reading partial numbers: 1.0f, 1.0/, and 1.0+

fix buffer overrun reading partial numbers: 1.0f, 1.0/, and 1.0+

* libguile/numbers.c (mem2decimal_from_point, mem2ureal, mem2complex):
  Fix a number of cases where, for invalid numbers, we could read past
  the end of the buffer. This happened in e.g. "1.0+", "1/" and "1.0f".
  But I couldn't figure out how to test for these, given that the
  behavior depended on the contents of uninitialized memory in the
  reader buffer. We'll just have to be happy with this.

Thanks to Kjetil S. Matheussen for the report.