[Object][COFF] Fix section name parsing error when the name field is not null-padded

Some object files produced by Mirosoft tools contain sections whose name field
is not fully null-padded at the end. Microsoft's dumpbin is able to print the
section name correctly, but this causes parsing errors with LLVM tools.

So far, this issue only seems to happen when the section name is longer than 8
bytes. In this case, the section name field contains a slash (/) followed by the
offset into the string table, but the name field is not fully null-padded at the
end.

Reviewed By: mstorsjo

Differential Revision: https://reviews.llvm.org/D127369