Handle `.bss`-only data segment

This commit fixes an assertion triggered by the fact that a segment
includes exclusively zero-initialized data (i.e., size on file is 0,
memory size is not). In this case LLVM detects the fact that the global
variable associated to the segment is composed exclusively composed by
0s and uses a `ConstantAggregateZero` as an initializer instead of a
`ConstantDataArray`.

Currently the solution is ignore that data, however, in the future it
might be beneficial to be able to read data from `.bss`, even if we just
have zeros there.

Thanks to Thorbjoern Schulz for reporting this bug.