ZJIT: Bail out of recursive compilation if we can't compile callee

Right now we just crash if we can't compile an ISEQ for any reason
(unimplemented in HIR, unimplemented in codegen, ...) and this fixes
that by bailing out.

zjit/src/codegen.rs

@@ -126,6 +126,9 @@ fn gen_iseq_entry_point(iseq: IseqPtr) -> *const u8 {
                 asm.ccall(callee_addr, vec![]);
             });
             branch_iseqs.extend(callee_branch_iseqs);
+        } else {
+            // Failed to compile the callee. Bail out of compiling this graph of ISEQs.
+            return std::ptr::null();
         }
     }