Fixed a crash when using mock with posix #1439

Job	Run time
Coverage (STP)	28m 48s
Docker	8m 33s
clang-format	25s
Coverage (Z3)	30m 17s
Coverage (Bitwuzla)	30m 56s
macOS	5m 13s
Linux (LLVM 14)	10m 41s
Linux (LLVM 13)	9m 53s
Linux (LLVM 12)	10m 23s
Linux (LLVM 11, Doxygen)	10m 39s
Linux (LLVM 10)	10m 21s
Linux (LLVM 9)	9m 32s
Linux (ASan)	48m 55s
Linux (UBSan)	50m 39s
Linux (MSan)	1h 16m 17s
Linux (Z3 only)	10m 9s
Linux (metaSMT)	20m 14s
Linux (STP master)	10m 48s
Linux (Bitwuzla only)	9m 25s
Linux (Latest klee-uclibc)	12m 21s
Linux (Asserts disabled)	9m 51s
Linux (No TCMalloc, optimised runtime)	30m 0s
	7h 24m 20s

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