I urge anybody using a C family language to become familiar with AddressSanitizer [1], a tool that (with support from the compiler) detects an impressive variety of memory bugs at runtime. The scenario you describe would result in the program immediately crashing with a read-after-free error, along with stack traces for the most recent calls to malloc and free that involved that address. It's remarkably useful and straightforward, and Apple's tooling has good support for it, along with Xcode integration [2].

[1] https://clang.llvm.org/docs/AddressSanitizer.html

[2] https://developer.apple.com/documentation/xcode/diagnosing-m...