hw/riscv: Don't add empty bootargs to device tree

Commit 7c28f4da20 ("RISC-V: Don't add NULL bootargs to device-tree")
tried to avoid adding *NULL* bootargs to device tree, but unfortunately
the changes were entirely useless, due to MachineState::kernel_cmdline
can't be NULL at all as the default value is given as an empty string.
(see hw/core/machine.c::machine_initfn()).

Note the wording of *NULL* bootargs is wrong. It can't be NULL otherwise
a segfault had already been observed by dereferencing the NULL pointer.
It should be worded as *empty" bootargs.

Fixes: 7c28f4da20 ("RISC-V: Don't add NULL bootargs to device-tree")
Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20220421055629.1177285-2-bmeng.cn@gmail.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

hw/riscv/microchip_pfsoc.c

@@ -571,7 +571,7 @@ static void microchip_icicle_kit_machine_init(MachineState *machine)
                                   "linux,initrd-end", end);
         }
 
-        if (machine->kernel_cmdline) {
+        if (machine->kernel_cmdline && *machine->kernel_cmdline) {
             qemu_fdt_setprop_string(machine->fdt, "/chosen",
                                     "bootargs", machine->kernel_cmdline);
         }

hw/riscv/sifive_u.c

@@ -511,7 +511,7 @@ static void create_fdt(SiFiveUState *s, const MemMapEntry *memmap,
     g_free(nodename);
 
 update_bootargs:
-    if (cmdline) {
+    if (cmdline && *cmdline) {
         qemu_fdt_setprop_string(fdt, "/chosen", "bootargs", cmdline);
     }
 }

hw/riscv/spike.c

@@ -177,7 +177,7 @@ static void create_fdt(SpikeState *s, const MemMapEntry *memmap,
     qemu_fdt_add_subnode(fdt, "/chosen");
     qemu_fdt_setprop_string(fdt, "/chosen", "stdout-path", "/htif");
 
-    if (cmdline) {
+    if (cmdline && *cmdline) {
         qemu_fdt_setprop_string(fdt, "/chosen", "bootargs", cmdline);
     }
 }

hw/riscv/virt.c

@@ -1004,7 +1004,7 @@ static void create_fdt(RISCVVirtState *s, const MemMapEntry *memmap,
     create_fdt_flash(s, memmap);
 
 update_bootargs:
-    if (cmdline) {
+    if (cmdline && *cmdline) {
         qemu_fdt_setprop_string(mc->fdt, "/chosen", "bootargs", cmdline);
     }
 }