CVE-2020-16592: Use-after-free in BFD merge.c during section merging

BFD (Binary File Descriptor library) contains a use-after-free vulnerability in its SEC_MERGE section handling code. When processing merged sections in merge_strings(), the code accesses section information structures (sec_merge_sec_info) that may have been freed or invalidated. This occurs because the first section in the merge chain is extracted and processed separately, but entries in the hash table that reference this section are not properly validated before access. A crafted ELF file with specially constructed merge sections can trigger access to freed memory during the linking process.