Allthough the size of MAX_MUX is equal to 4 and likely will never
change, this patch changes type of constant to unsigned long to
be on the safe side.

Also add a static compile check that MAX_MUX never bigger than
`sizeof(d->mux_bitset)`.

Signed-off-by: Roman Penyaev <r.peniaev@gmail.com>
Reviewed-by: "Marc-André Lureau" <marcandre.lureau@redhat.com>
Cc: Peter Maydell <peter.maydell@linaro.org>:
Reviewed-by: Clément Mathieu--Drif <clement.mathieu--drif@eviden.com>
Cc: qemu-devel@nongnu.org
---
 chardev/char-mux.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/chardev/char-mux.c b/chardev/char-mux.c
index bda5c45e6058..355520714979 100644
--- a/chardev/char-mux.c
+++ b/chardev/char-mux.c
@@ -316,6 +316,8 @@ bool mux_chr_attach_frontend(MuxChardev *d, CharBackend *b,
 {
     unsigned int bit;
 
+    QEMU_BUILD_BUG_ON(MAX_MUX > sizeof(d->mux_bitset));
+
     bit = find_next_zero_bit(&d->mux_bitset, MAX_MUX, 0);
     if (bit >= MAX_MUX) {
         error_setg(errp,
@@ -325,7 +327,7 @@ bool mux_chr_attach_frontend(MuxChardev *d, CharBackend *b,
         return false;
     }
 
-    d->mux_bitset |= (1 << bit);
+    d->mux_bitset |= (1ul << bit);
     d->backends[bit] = b;
     *tag = bit;
 
@@ -341,7 +343,7 @@ bool mux_chr_detach_frontend(MuxChardev *d, unsigned int tag)
         return false;
     }
 
-    d->mux_bitset &= ~(1 << bit);
+    d->mux_bitset &= ~(1ul << bit);
     d->backends[bit] = NULL;
 
     return true;
-- 
2.34.1