buffer overflow detected when adding 10 million or more columns

[Wert5]

When adding the 10 millionth column to a CoinModel I get the following error message:
*** buffer overflow detected *** : ./solver terminated
A stack trace shows that the error occurs during CoinModel::addColumn(int, int const*, double const*, double, double, double, char const*, bool) () from /usr/lib/x86_64-linux-gnu/libCoinUtils.so.3
The next function on the stack trace was ___sprintf_chk which suggests that the problem is an overflow writing to a string.

Examining the code, I believe the issue is the format limitation of 7 characters for the number when creating a default name in addColumn. This makes sense because 10000000 is the first integer that requires 8 characters in a string.

CoinUtils/src/CoinModel.cpp

Lines 929 to 935 in ee7c023

	if (name) {
	columnName_.addHash(numberColumns_, name);
	} else if (!noNames_) {
	char name[9];
	sprintf(name, "c%7.7d", numberColumns_);
	columnName_.addHash(numberColumns_, name);
	}

Increasing this limit from 7 characters to 10 characters would allow up to 9.999 * 10^9 = almost 10 billion variables, which is way more than anyone could practically solve.

Currently, I can work around this issue by setting noNames to true in the CoinModel constructor. I have confirmed that setting noNames to true prevents the error.

Although I understand that 10 million variables may be more variables than Coin was designed for, it solves in a reasonable time. My program, which uses Coin to solve 2 problems with over 10 million variables, finishes running in 12 minutes on my laptop.

[jjhforrest]

Not wishing to disturb the code too much, I think the best thing to do is leave maximum at 8 characters when adding a row/column without a name. I will add a function to CoinUtils to create an 8 character name for a row or column. If >10000000 it will start using characters as well - this will easily allow up to 2**31 which is maximum number of rows/columns. In other places the coding is slightly different and may not have same bug - but if the error is found in other places we can just slot in that function.

[jjhforrest]

Put change into master. The names are not elegant - I use a - z and pad out with 0.