[gdb/testsuite, Tcl 9.0] Fix uninitialized string in gdb.fortran/function-calls.f90

With Tcl 9.0 and test-case gdb.fortran/function-calls.f90 I run into:
...
(gdb) run ^M
Starting program: function-calls ^M
[Thread debugging using libthread_db enabled]^M
Using host libthread_db library "/lib64/libthread_db.so.1".^M
             (2.09999990,3.29999995)^M
           4           5^M
          23^M
             (2.09999990,3.29999995)^M
   3.40000010    ^M
           6^M
 ^K^@^@^@^@^@^@^@ERROR: i_read(spawn_id fd=9): invalid or incomplete multibyte or wide character
...

The problem seems to be the printing of an uninitialized string.

Fix this by initializing the string.

Approved-By: Tom Tromey <tom@tromey.com>

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=33948