gdb: fix potentially uninitialised variable

In commit:

  commit 037d7135
  Date:   Mon Nov 16 11:36:56 2020 +0000

      gdb: improve command completion for 'print', 'x', and 'display'

A potential use of an uninitialised variable was introduced.  This is
fixed in this commit.

Previously when analysing /FMT strings for tab completion we
considered two possibilities, either the user has typed '/', or the
user has typed '/' followed by an alpha-numeric character, as these
are the only valid FMT string characters.

This meant that if the user type, for example '/@' and then tried to
tab complete gdb would use an uninitialised variable.

Currently only the first character after the '/' is checked to see if
it is alpha-numeric, so if a user typed '/x@@' then gdb would be happy
to treat this as a FMT string.

Given the goal of this change was primarily to allow tab completion of
symbols later in the command when a /FMT was used then I decided to
just make the /FMT skipping less smart.  Now any characters after the
'/' up to the first white space, will be treated as a FMT string.

gdb/ChangeLog:

	* printcmd.c (skip_over_slash_fmt): Reorder code to ensure in_fmt
	is always initialized.