bitswap(session): fix slice prealloc

[Wondertan]

Fixes the following:

panic: runtime error: makeslice: cap out of range

goroutine 632 [running]:
github.com/ipfs/boxo/bitswap/client/internal/session.(*sessionWants).GetNextWants(0xc004e06060)
	/go/pkg/mod/github.com/!wondertan/[email protected]/bitswap/client/internal/session/sessionwants.go:64 +0x98
github.com/ipfs/boxo/bitswap/client/internal/session.(*Session).wantBlocks(0xc004e06000, {0x3a8ff70, 0xc000531040}, {0xc03d3b6908?, 0x80?, 0x80?})
	/go/pkg/mod/github.com/!wondertan/[email protected]/bitswap/client/internal/session/session.go:469 +0x9a
github.com/ipfs/boxo/bitswap/client/internal/session.(*Session).run(0xc004e06000, {0x3a8ff70, 0xc000531040})
	/go/pkg/mod/github.com/!wondertan/[email protected]/bitswap/client/internal/session/session.go:317 +0x34b
created by github.com/ipfs/boxo/bitswap/client/internal/session.New in goroutine 283
	/go/pkg/mod/github.com/!wondertan/[email protected]/bitswap/client/internal/session/session.go:176 +0x659

Introduced in #692

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 60.31%. Comparing base (2f2c9dd) to head (6932e99).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
bitswap/client/internal/session/sessionwants.go	0.00%	0 Missing and 1 partial ⚠️

@@            Coverage Diff             @@
##             main     #702      +/-   ##
==========================================
- Coverage   60.36%   60.31%   -0.05%     
==========================================
  Files         243      243              
  Lines       31025    31025              
==========================================
- Hits        18727    18714      -13     
- Misses      10630    10639       +9     
- Partials     1668     1672       +4     

Files with missing lines	Coverage Δ
bitswap/client/internal/session/sessionwants.go	88.99% <0.00%> (ø)

... and 13 files with indirect coverage changes

[gammazero]

Thank you for this fix!