[gdb/tui] Fix length of status line string

In commit 5d10a2041eb ("gdb: add string_file::release method") this was added:
...
+  std::string string_val = string.release ();
...
without updating subsequent uses of string.size (), which returns 0 after the
string.release () call.

Fix this by:
- using string_val.size () instead of string.size (), and
- adding an assert that would have caught this regression.

Tested on x86_64-linux.

Reviewed-By: Simon Marchi <simon.marchi@efficios.com>
PR tui/30389
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30389