fix: NULL dereference in bios(4)

[Jan Klemkow]

Hi,

fixstring() can return NULL and it does on one of my machines.
Here is s fix that checks for NULL and return an empty string.

ok?

bye,
Jan

Index: arch/amd64/amd64/bios.c
===================================================================
RCS file: /cvs/src/sys/arch/amd64/amd64/bios.c,v
retrieving revision 1.38
diff -u -p -r1.38 bios.c
--- arch/amd64/amd64/bios.c     15 Jul 2019 00:35:10 -0000      1.38
+++ arch/amd64/amd64/bios.c     19 Jul 2019 08:56:27 -0000
@@ -144,7 +144,9 @@ bios_attach(struct device *parent, struc
                                    fixstring(scratch));
                        if ((smbios_get_string(&bios, sb->release,
                            scratch, sizeof(scratch))) != NULL) {
-                               strlcpy(smbios_bios_date, fixstring(scratch),
+                               strlcpy(smbios_bios_date,
+                                   fixstring(scratch) == NULL ? "" :
+                                       fixstring(scratch),
                                    sizeof(smbios_bios_date));
                                printf(" date %s", fixstring(scratch));
                        }