If a number string has leading spaces and/or minus, then calculating
index into the aBuf as the difference between p and pStart gives
wrong (too big) number. This asserts in debug builds, when e.g. an
assignment is made in a BASIC macro:

  Dim f As Double
  f = " -12.20"

"include/rtl/ustrbuf.hxx:490:
sal_Unicode &rtl::OUStringBuffer::operator[](sal_Int32):
Assertion `index >= 0 && index < pData->length' failed."

This affects, e.g., https://gerrit.libreoffice.org/56610 (see
https://ci.libreoffice.org/job/gerrit_linux_clang_dbgutil/9527/consoleFull#1820989527d893063f-7f3d-4b7e-b56f-4e0f225817cd)

Change-Id: I14654166be721907e2a26ea6f4091f203a9437d7
Reviewed-on: https://gerrit.libreoffice.org/56611Reviewed-by: Mike Kaganski <mike.kaganski@collabora.com>
Tested-by: Jenkins