fuzzing: Check that `Intbuf` was successfully allocated #225

Job	Run time
Code-checks	2m 11s
Windows-cmake-build	29s
Linux-build (debug-sanitizers)	35s
Macos-build	42s
Linux-build (no-mnemonic)	15s
Windows-msbuild	3m 19s
Linux-build (release)	2m 55s
	10m 26s

Provide feedback