fix(path): check return value of append_path() (#28309)

If the filename passed to vim_FullName() is a relative directory, and
does not exist, it is appended to the current working directory. Since
the return value of append_path() was ignored, and if the buffer length
was too small to fit getcwd() + dirname(filename), it would still try to
append the basename(filename).

This was manifesting as a failure in test/unit/path_spec.lua in:
    itp('fails and uses filename if given filename contains non-existing directory', ..

This failure occurs when running the tests from directory with a short
path such as: /work/src/nv

    test/unit/path_spec.lua:420: Expected objects to be the same.
    Passed in:
    (string) '/work/src/nv/test.file'
    Expected:
    (string) 'non_existing_dir/test.file'

This return value for the second call to append_path() to append
basename(filename) was checked, and this is where it would fail for
normal / longer getcwd()s.

src/nvim/path.c

@@ -2301,7 +2301,9 @@ int path_full_dir_name(char *directory, char *buffer, size_t len)
       retval = FAIL;
     } else {
       xstrlcpy(buffer, old_dir, len);
-      append_path(buffer, directory, len);
+      if (append_path(buffer, directory, len) == FAIL) {
+        retval = FAIL;
+      }
     }
   } else if (os_dirname(buffer, len) == FAIL) {
     // Do not return immediately since we are in the wrong directory.