gdb, testsuite: fix TCL error in 'gdb.base/structs.exp'

A failure of 'runto_main' in 'start_structs_test' results in a TCL
error.  The return value of 'start_structs_test' function is evaluated
inside an if conditional clause, which expects a boolean value.  Return
'-1' on failure to avoid the error.

Reviewed-By: Keith Seitz <keiths@redhat.com>
Approved-By: Tom Tromey <tom@tromey.com>