gdb/source.c: Fix undefined behaviour dereferencing empty string

gdb/source.c: Fix undefined behaviour dereferencing empty string

When a source file's dirname is solely made up of directory separators
we end up trying to dereference the last character of an empty string
with std::string::back, which results in undefined behaviour. A typical
use case where this can happen is when the root directory "/" is used as
a compilation directory.

With libstdc++.so.6.0.28 we get no out-of-bounds checks and the byte
preceding the storage of the empty string is returned. The character
value of this byte depends on heap implementation and usage, but when
this byte happens to hold the value of the directory separator character
we go on to call std::string::pop_back on the empty string which results
in an out_of_range exception which terminates GDB.

Fix this by using path_join. prepare_path_for_appending ensures that the
filename component is relative.

The testsuite has been run before and after the change and no
regressions were found.