Fix UB: memcpy requires valid pointers even with count=0 (#1198)

* Fix UB: memcpy requires valid pointers even with count=0

ASAN detected undefined behavior in nb_bound_method_vectorcall where
memcpy was called with a potentially NULL args_in pointer.

Root Cause:
Per C standard section 7.24.1 paragraph 2, pointer arguments to memcpy
must have valid values even when copying zero bytes. NULL is not a valid
pointer value.

When args_in can be NULL:
According to Python vectorcall protocol PEP 590, when a bound method is
called with zero positional arguments, the args_in pointer may be NULL.
This is valid Python behavior - Python does not allocate an array when
there are no arguments to pass.

Why the guard checks size > 1:
The code calculates size = nargs + 1 for the implicit self argument,
then copies size - 1 elements from args_in. When nargs = 0:
- size = 1
- We attempt to copy size - 1 = 0 elements
- Even though copying 0 bytes, calling memcpy with NULL violates C standard

Fix:
Guard the memcpy to only execute when there are actual arguments to copy,
i.e., when size > 1 and args_in is not NULL.

This resolves ASAN failures in projects using batch array operations with
nanobind bindings, where methods are frequently called with no arguments.

* Remove redundant NULL check per code review

Author: jeongseok-meta

src/nb_func.cpp

@@ -1096,7 +1096,8 @@ static PyObject *nb_bound_method_vectorcall(PyObject *self,
             alloc = true;
         }
 
-        memcpy(args + 1, args_in, sizeof(PyObject *) * (size - 1));
+        if (size)
+            memcpy(args + 1, args_in, sizeof(PyObject *) * (size - 1));
     }
 
     args[0] = mb->self;