[CORE] modules: Fix potential NULL pointer de-reference

Coverity scan detected if setup_load_info() function did not find
valid symbol table section in loadable module then it will cause
NULL pointer de-reference to string table in layout_symtab() via
alloc_and_load_sections().

This issue can be catastrophic for Xvisor if someone tries to load
invalid .xo loadable module.

This patch fixes this issue.

Signed-off-by: Anup Patel <anup@brainfault.org>