drm/amdgpu: Fix vbios build number parsing logic

It's not necessary that the build string and atom header section has a
difference of 32 bytes. Use the remaining bytes in the section as copy
limit.

Fixes: d6fa80266178 ("drm/amdgpu: Add vbios build number interface")
Signed-off-by: Lijo Lazar <[e-mail]>
Acked-by: Alex Deucher <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>

drivers/gpu/drm/amd/amdgpu/atom.c

···
 {
 	unsigned char *atom_rom_hdr;
 	unsigned char *str;
-	uint16_t base;
+	uint16_t base, len;
 
 	base = CU16(ATOM_ROM_TABLE_PTR);
 	atom_rom_hdr = CSTR(base);
···
 	while (str < atom_rom_hdr && *str++)
 		;
 
-	if ((str + STRLEN_NORMAL) < atom_rom_hdr)
-		strscpy(ctx->build_num, str, STRLEN_NORMAL);
+	len = min(atom_rom_hdr - str, STRLEN_NORMAL);
+	if (len)
+		strscpy(ctx->build_num, str, len);
 }
 
 struct atom_context *amdgpu_atom_parse(struct card_info *card, void *bios)