nir/builder: Fix compilation with gcc-13 when tsan is enabled

../src/compiler/nir/nir_builder.h: In function ‘nir_build_deref_follower’:
../src/compiler/nir/nir_builder.h:1607:1: error: control reaches end of non-void function [-Werror=return-type]
 1607 | }

Fixes: 4a4e175738
    nir: Support deref instructions in lower_var_copies

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/27345>
(cherry picked from commit 0ab3b3c641)

.pick_status.json

@@ -634,7 +634,7 @@
         "description": "nir/builder: Fix compilation with gcc-13 when tsan is enabled",
         "nominated": true,
         "nomination_type": 1,
-        "resolution": 0,
+        "resolution": 1,
         "main_sha": null,
         "because_sha": "4a4e1757381c65653fd6a81cf4e969ddff926ed5",
         "notes": null

src/compiler/nir/nir_builder.h

@@ -1594,6 +1594,7 @@ nir_build_deref_follower(nir_builder *b, nir_deref_instr *parent,
    default:
       unreachable("Invalid deref instruction type");
    }
+   return NULL;
 }
 
 static inline nir_def *