Abort creating wrapper fn for multiple inner fns

This discovers another class of mis-trans where we wrap multiple native functions into a single
wrapper, which is wrong.

Author: nagisa

src/librustc_trans/trans/foreign.rs

@@ -642,6 +642,11 @@ pub fn trans_rust_fn_with_foreign_abi<'a, 'tcx>(ccx: &CrateContext<'a, 'tcx>,
         //         return r;
         //     }
 
+        if llvm::LLVMCountBasicBlocks(llwrapfn) != 0 {
+            ccx.sess().bug("wrapping a function inside non-empty wrapper, most likely cause is \
+                           multiple functions being wrapped");
+        }
+
         let ptr = "the block\0".as_ptr();
         let the_block = llvm::LLVMAppendBasicBlockInContext(ccx.llcx(), llwrapfn,
                                                             ptr as *const _);