lib/glob: initialize back_str to silence uninitialized variable warning

lib/glob: initialize back_str to silence uninitialized variable warning

back_str is only used when back_pat is non-NULL, and both are always set
together, so it is safe in practice.  Initialize back_str to NULL to make
this safety invariant explicit and silence compiler/static analysis
warnings.

Link: https://lkml.kernel.org/r/20260312215249.50165-1-objecting@objecting.org
Signed-off-by: Josh Law <objecting@objecting.org>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>