MC: ensure that we have a section before accessing it

We would attempt to access the symbol section without ensuring that the symbol
was not absolute.  When the assembler referenced relocation is not evaluated to
the absolute, but when we record the relocation, we would query the section.
Because the symbol is absolute, it does not have a section associated with it,
triggering an assertion.  Just be more careful about the access of the section.

Addresses PR31064!

llvm-svn: 287619