linux-user/mmap.c: Avoid choosing NULL as start address

mmap() is required by the linux kernel ABI and POSIX to return a
non-NULL address when the implementation chooses a start address for the
mapping.

The current implementation of mmap_find_vma_reserved() can return NULL
as start address of a mapping which leads to subsequent crashes inside
the guests glibc, e.g. output of qemu-arm-static --strace executing a
test binary stx_test:

    1879 mmap2(NULL,8388608,PROT_READ|PROT_WRITE,MAP_PRIVATE|MAP_ANONYMOUS|0x20000,-1,0) = 0x00000000
    1879 write(2,0xf6fd39d0,79) stx_test: allocatestack.c:514: allocate_stack: Assertion `mem != NULL' failed.

This patch fixes mmap_find_vma_reserved() by skipping NULL as start
address while searching for a suitable mapping start address.

CC: Riku Voipio <riku.voipio@iki.fi>
CC: Laurent Vivier <laurent@vivier.eu>
CC: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Maximilian Riemensberger <riemensberger@cadami.net>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <1515286904-86418-1-git-send-email-riemensberger@cadami.net>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>
