gdb, testsuite: Fix return value in gdb.base/foll-fork.exp

gdb, testsuite: Fix return value in gdb.base/foll-fork.exp

In a remote testing setup, I saw this error:

~~~
(gdb) FAIL: gdb.base/foll-fork.exp: check_fork_catchpoints: runto: run to main
ERROR: tcl error sourcing gdb/gdb/testsuite/gdb.base/foll-fork.exp.
ERROR: expected boolean value but got ""
    while executing
"if { ![check_fork_catchpoints] } {
    untested "follow-fork not supported"
    return
}"
    (file "gdb/gdb/testsuite/gdb.base/foll-fork.exp" line 434)
    invoked from within
"source gdb/gdb/testsuite/gdb.base/foll-fork.exp"
    ("uplevel" body line 1)
    invoked from within
"uplevel #0 source gdb/gdb/testsuite/gdb.base/foll-fork.exp"
    invoked from within
"catch "uplevel #0 source $test_file_name""
Remote debugging from host 172.0.1.3, port 37766
Killing process(es): 1171
Quit
~~~

The actual reason for this were some connection problems. Though the
function check_fork_catchpoints shouldn't return an empty string, especially
as it promises to always return 0 or 1. Fix that.

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