lib: remove back_str initialization

Clang static analysis reports this false positive
glob.c:48:32: warning: Assigned value is garbage
  or undefined
  char const *back_pat = NULL, *back_str = back_str;
                                ^~~~~~~~   ~~~~~~~~

back_str is set after back_pat and it's use is protected by the !back_pat
check.  It is not necessary to initialize back_str, so remove the
initialization.

Link: https://lkml.kernel.org/r/20220402131546.3383578-1-trix@redhat.com
Signed-off-by: Tom Rix <trix@redhat.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Cc: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>