mesa: fix warning about possibly undefined var in GLSL compiler

(cherry picked from commit 724ed5b84bc2491da244d091de889d5b35503b5d)
diff --git a/src/mesa/shader/slang/slang_emit.c b/src/mesa/shader/slang/slang_emit.c
index 6d36f70..35d3231 100644
--- a/src/mesa/shader/slang/slang_emit.c
+++ b/src/mesa/shader/slang/slang_emit.c
@@ -310,24 +310,22 @@
       dst->WriteMask = swizzle_to_writemask(swizzle);
    }
    else {
-      GLuint writemask;
       switch (size) {
       case 1:
-         writemask = WRITEMASK_X << GET_SWZ(st->Swizzle, 0);
+         dst->WriteMask = WRITEMASK_X << GET_SWZ(st->Swizzle, 0);
          break;
       case 2:
-         writemask = WRITEMASK_XY;
+         dst->WriteMask = WRITEMASK_XY;
          break;
       case 3:
-         writemask = WRITEMASK_XYZ;
+         dst->WriteMask = WRITEMASK_XYZ;
          break;
       case 4:
-         writemask = WRITEMASK_XYZW;
+         dst->WriteMask = WRITEMASK_XYZW;
          break;
       default:
          ; /* error would have been caught above */
       }
-      dst->WriteMask = writemask;
    }
 
    dst->RelAddr = relAddr;