refactor(ffi): return null ptr instead of aborting in C API (#2478)

Make C API functions that return pointers return null in case of a
panic, instead of aborting.

Add ffi_fn! macro rules that enable default error values to be returned
by writing "?= <value>" after an ffi function's body.

src/ffi/mod.rs

@@ -92,5 +92,5 @@ ffi_fn! {
     /// Returns a static ASCII (null terminated) string of the hyper version.
     fn hyper_version() -> *const libc::c_char {
         VERSION_CSTR.as_ptr() as _
-    }
+    } ?= std::ptr::null()
 }