[settings] Ensure fetch_string_setting() returns a NUL-terminated string

This fixes a regression introduced in commit 612f4e7:

  [settings] Avoid returning uninitialised data on error in fetch_xxx_setting()

in which the memset() was moved from fetch_string_setting() to
fetch_setting(), in order that it would be useful for non-string
setting types.  However, this neglects to take into account the fact
that fetch_string_setting() shrinks its buffer by one byte (to allow
for the NUL) before calling fetch_setting().

Restore the memset() in fetch_string_setting(), so that the
terminating NUL is guaranteed to actually be a NUL.

src/core/settings.c

  */
 int fetch_string_setting ( struct settings *settings, struct setting *setting,
 			   char *data, size_t len ) {
+	memset ( data, 0, len );
 	return fetch_setting ( settings, setting, data,
 			       ( ( len > 0 ) ? ( len - 1 ) : 0 ) );
 }