nv50/ir: Initialize BuildUtil member tail.

Fix defect reported by Coverity Scan.

Uninitialized scalar field (UNINIT_CTOR)
uninit_member: Non-static class member tail is not initialized in
this constructor nor in any functions that it calls.

Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10561>

src/gallium/drivers/nouveau/codegen/nv50_ir_build_util.cpp

@@ -44,6 +44,8 @@ BuildUtil::init(Program *prog)
    bb = NULL;
    pos = NULL;
 
+   tail = false;
+
    memset(imms, 0, sizeof(imms));
    immCount = 0;
 }