linux-user/elfload.c: Don't memset(NULL..) if malloc() failed

If a malloc() in copy_elf_strings() failed we would call memset()
before the "did malloc fail?" check. Fix this by moving to the
glib alloc/free routines for this memory so we can use g_try_malloc0
rather than having a separate memset(). Spotted by Coverity (see
bug 887883).

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>