C library/time: fix wrong is-null test

[tautschnig]

The assignment should happen if the pointer is non-null, not when it is
null. Also test this library function.

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
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[codecov]

Codecov Report

Merging #5655 (c6f1ed5) into develop (4bd3c2f) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #5655   +/-   ##
========================================
  Coverage    69.41%   69.41%           
========================================
  Files         1243     1243           
  Lines       100612   100612           
========================================
  Hits         69842    69842           
  Misses       30770    30770           

Flag	Coverage Δ
cproversmt2	43.15% <ø> (ø)
regression	66.31% <ø> (ø)
unit	32.26% <ø> (ø)

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[martin-cs]

Wow! This is an old bug. Makes me wonder if it has ever ben used!