opkg: Protect add-exclude.patch from malloc failure

opkg: Protect add-exclude.patch from malloc failure

In the code added by add-exclude.patch, the return values of malloc and realloc
were not checked before being dereferenced.

In opkg we can use xmalloc and xrealloc instead of malloc and realloc. These
functions terminate the program instead of returning NULL if memory allocation
fails.

(From OE-Core rev: 64048d45d5c52d354396e357e765f0fd8b5e56fd)

Signed-off-by: Paul Barker <paul@paulbarker.me.uk>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>