Fix double-free bug in stdlib models with --malloc-fail-null

[hannes-steffenhagen-diffblue]

If realloc results in a larger memory block the old one needs to be
freed... but this should only happen if the allocation of the new block
succeeds. The old model didn't properly check for that, this is now
fixed.

This should fix #5511

(Linked to wrong issue before)

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[chrisr-diffblue]

Good catch!

[codecov]

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@@            Coverage Diff             @@
##             develop    #5518   +/-   ##
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  Coverage           ?   68.40%           
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  Files              ?     1187           
  Lines              ?    98090           
  Branches           ?        0           
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  Hits               ?    67102           
  Misses             ?    30988           
  Partials           ?        0           

Flag	Coverage Δ
#cproversmt2	42.97% <ø> (?)
#regression	65.55% <ø> (?)
#unit	32.25% <ø> (?)

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[piotr-grabalski]

Looks good!

[thomasspriggs]

👍