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: 5ab23c79 ("modpost: Create modalias for builtin modules")
Signed-off-by: Hugh Dickins <hughd@google.com>
Reviewed-by: Nicolas Schier <nsc@kernel.org>
Link: https://patch.msgid.link/4590a243-0a7e-b7e6-e2d3-cd1b41a12237@google.com


Signed-off-by: Nathan Chancellor <nathan@kernel.org>