flaw in b2::bind::operator+ fixed

[paolopas]

Proposed changes

I was snooping around and found this problem on operator+, but it must not be used much otherwise it would have come up before.

Types of changes

What types of changes does this code introduce?

• [x ] Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Documentation content changes
• Other (please describe):

Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your code.

• I searched the discussions
• I searched the closed and open issues
• I read the contribution guidelines
• I added myself to the copyright attributions for significant changes
• I checked that tests pass locally with my changes
• I added tests that prove my fix is effective or that my feature works
• I added necessary documentation (if appropriate)

Further comments

I spent more time reading this form than the source header