lib/string.c: check for kmalloc() failure

lib/string.c: check for kmalloc() failure

This is mostly to keep the number of static checker warnings down so we
can spot new bugs instead of them being drowned in noise.  This function
doesn't return normal kernel error codes but instead the return value is
used to display exactly which memory failed.  I chose -1 as hopefully
that's a helpful thing to print.

Link: http://lkml.kernel.org/r/20170817115420.uikisjvfmtrqkzjn@mwanda
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Matthew Wilcox <mawilcox@microsoft.com>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Kees Cook <keescook@chromium.org>
Cc: Bjorn Helgaas <bhelgaas@google.com>
Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Cc: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Cc: Daniel Micay <danielmicay@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>