Malloc Q rather than from stack in multi-merger

[jeromekelleher]

Fixes segfault reported in #2307

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 6 lines in your changes missing coverage. Please review.

Project coverage is 90.99%. Comparing base (657fc8d) to head (6ac820a).

Files	Patch %	Lines
lib/msprime.c	50.00%	4 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2308      +/-   ##
==========================================
- Coverage   91.03%   90.99%   -0.05%     
==========================================
  Files          20       20              
  Lines       12023    12035      +12     
  Branches     2439     2441       +2     
==========================================
+ Hits        10945    10951       +6     
- Misses        595      599       +4     
- Partials      483      485       +2     

Flag	Coverage Δ
C	90.99% <50.00%> (-0.05%)	⬇️
python	98.70% <ø> (ø)

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