Given SwColumn::m_nWish is documented as "Desired width, borders included."
(sw/inc/fmtclds.hxx), it appears that the invariant

  GetWishWidth() >= GetLeft() + GetRight()

should always hold for SwColumn instances.

However, during UITest_writer_tests4's test_tdf113284 loading of
sw/qa/uitest/writer_tests/data/tdf113284.odt, it appears that some five-column
table is created for which initially all SwColumns'

  GetWishWidth() = 13107
  GetLeft() = 144 (except for first SwColumn, where it's 0)
  GetRight() = 144 (except for last SwColumn, where it's 0)

and later FitToActualSize is called reducing each SwColunn to

  GetWishWidth() = 286

so that for most SwColumns (except first and last), GetLeft() + GetRight() = 288
would exceed GetWishWidth().  And then (still during loading of the test's
document), SwFormatCol::CalcPrtColWidth (sw/source/core/layout/atrfrm.cxx) is
called for such a problematic SwColumn, trying to subtract from its
GetWishWidth() (the result of SwFormatCol::CalcColWidth) both GetLeft() and
GetRight(), and store that into sal_uInt16 nRet.  Which triggers Clang's
-fsanitize=implicit-signed-integer-truncation "runtime error: implicit
conversion from type 'int' of value -2 (32-bit, signed) to type 'sal_uInt16'
(aka 'unsigned short') changed the value to 65534 (16-bit, unsigned)".

So make sure that FitToAcutalSize upholds that presumed SwColumns invariant,
shrinking the borders as necessary.

Change-Id: I3f6fc07e65a0fc5d5f58c99f1dcdea4467b825f0
Reviewed-on: https://gerrit.libreoffice.org/68463
Tested-by: Jenkins
Reviewed-by: Miklos Vajna <vmiklos@collabora.com>