[gdb/testsuite] Fix gdb.base/style.exp with stub-termcap

[gdb/testsuite] Fix gdb.base/style.exp with stub-termcap

When running test-case gdb.base/style.exp with a gdb build using
stub-termcap.c, we run into:
...
(gdb) PASS: gdb.base/style.exp: all styles enabled: frame when width=20
^M<et width 30^M
(gdb) FAIL: gdb.base/style.exp: all styles enabled: set width 30
...

The problem is that we're trying to issue the command "set width 30" while
width is set to 20, which causes horizontal scrolling.

Fix this by resetting the width to 0 before issuing the "set width 30"
command.

Tested on x86_64-linux.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=24582