gprofng SIGSEGV when processing unusual dwarf

[Gilles DUBOSCQ]

Hi Vladimir,

I saw you posted a patch for this. A few remarks:
* Regarding the NO_STMT_LIST, i'm not sure what it the correct/portable way of defining uint64_t constants, i used a ULL literal for my tests but maybe UINT64_MAX should be used instead? I'm not sure exactly if this is available in all the environments where gprofng is built.
* Regarding the change in DwrCU::parse_cu_header, i think Dwarf_ref will also return 0 when the attribute is not found, what i was trying to achieve in my tests was for stmt_list_offset to be kept at its default NO_STMT_LIST value if the attribute is absent.

Best regards,
 Gilles