Pacify gcc -Wanalyzer-null-dereference

This is for gcc 12.2.1 20220819 (Red Hat 12.2.1-2) x86-64
when Emacs is configured with --enable-gcc-warnings.
* src/buffer.c (Fmove_overlay): Prefer BASE_EQ to EQ in a place
where they’re equivalent because the only symbol allowed here is nil.

src/buffer.c

@@ -3619,7 +3619,7 @@ buffer.  */)
       o_end = OVERLAY_END (overlay);
     }
 
-  if (! EQ (buffer, obuffer))
+  if (! BASE_EQ (buffer, obuffer))
     {
       if (! NILP (obuffer))
         remove_buffer_overlay (XBUFFER (obuffer), XOVERLAY (overlay));