gas: avoid dangling pointers into freed memory

The oss-fuzz gas fuzzer is quite broken in that it doesn't
reinitialise all gas and bfd static variables between runs.  Since gas
naughtily modifies bfd_und_section and bfd_abs_section those bfd
statics can hold pointers into freed memory between runs.
This patch fixes oss-fuzz issue 398060144.
1 file changed