Fix crash when cu_comp_dir parameter to DWARFDebugLine::ParseSupportFiles() is an empty string

[ted]

Summary

If cu_comp_dir is "", this will crash as it tries to access index -1:

fullpath = cu_comp_dir;

if (*fullpath.rbegin() != '/')

[clayborg]

Looks good. I would also file a bug on the compiler that generated and empty DW_AT_comp_dir as this is just bad DWARF. Valid, but bad.

[ted]

Handled by another patch

source/Plugins/SymbolFile/DWARF/DWARFDebugLine.cpp

while (offset < end_prologue_offset)
                     support_files.Append(FileSpec (fullpath.c_str(), false));
                 else
                     support_files.Append(FileSpec (path, false));
             }
             else
             {
                 if (dir_idx > 0 && dir_idx < include_directories.size())
                 {
-                    if (cu_comp_dir && include_directories[dir_idx][0] != '/')
+                    if (cu_comp_dir && cu_comp_dir[0] != '\0' && include_directories[dir_idx][0] != '/')
                     {
                         fullpath = cu_comp_dir;
 
                         if (*fullpath.rbegin() != '/')
                             fullpath += '/';
                         fullpath += include_directories[dir_idx];
 
                     }