chardev/char-fe: Allow NULL chardev in qemu_chr_fe_init()

All the functions in char-fe.c handle the CharBackend
having a NULL Chardev pointer, which means that the
backend exists but is not connected to anything. The
exception is qemu_chr_fe_init(), which will crash if
passed a NULL Chardev pointer argument. This can happen
for various boards if they're started with 'nodefaults':
 arm-softmmu/qemu-system-arm -S -nodefaults -M cubieboard
 riscv32-softmmu/qemu-system-riscv32 -nodefaults -M sifive_e

Make qemu_chr_fe_init() accept a NULL chardev. This allows
UART models to handle NULL chardev properties without
generally needing to special case them or to manually
create a NullChardev.
Reported-by: NThomas Huth <thuth@redhat.com>
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Message-Id: <20180323152948.27048-1-peter.maydell@linaro.org>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

chardev/char-fe.c

@@ -198,19 +198,21 @@ bool qemu_chr_fe_init(CharBackend *b, Chardev *s, Error **errp)
 {
     int tag = 0;
 
-    if (CHARDEV_IS_MUX(s)) {
-        MuxChardev *d = MUX_CHARDEV(s);
+    if (s) {
+        if (CHARDEV_IS_MUX(s)) {
+            MuxChardev *d = MUX_CHARDEV(s);
+
+            if (d->mux_cnt >= MAX_MUX) {
+                goto unavailable;
+            }
 
-        if (d->mux_cnt >= MAX_MUX) {
+            d->backends[d->mux_cnt] = b;
+            tag = d->mux_cnt++;
+        } else if (s->be) {
             goto unavailable;
+        } else {
+            s->be = b;
         }
-
-        d->backends[d->mux_cnt] = b;
-        tag = d->mux_cnt++;
-    } else if (s->be) {
-        goto unavailable;
-    } else {
-        s->be = b;
     }
 
     b->fe_open = false;