modpost: Initialize builtin_modname to stop SIGSEGVs

Segmentation fault ./scripts/mod/modpost -o vmlinux.symvers vmlinux.o
stops the kernel build. It comes when write_vmlinux_export_c_file()
tries to buf_printf alias->builtin_modname. malloc'ed memory is not
necessarily zeroed. NULL new->builtin_modname before adding to aliases.

Fixes: 5ab23c7923a1 ("modpost: Create modalias for builtin modules")
Signed-off-by: Hugh Dickins <[e-mail]>
Reviewed-by: Nicolas Schier <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Nathan Chancellor <[e-mail]>

scripts/mod/file2alias.c

···
 		}
 	}
 
+	new->builtin_modname = NULL;
 	list_add_tail(&new->node, &mod->aliases);
 }