tools: fix NULL_AFTER_DEREF in image-host.c

tools: fix NULL_AFTER_DEREF in image-host.c

Report of the static analyzer:
1. NULL_AFTER_DEREF Pointer 'str', which is dereferenced at
   image-host.c:688 by calling function 'strdup', is compared to a NULL
   value at image-host.c:691.
2. NULL_AFTER_DEREF Pointer 'list', which is dereferenced at
   image-host.c:689, is compared to a NULL value at image-host.c:691.

Corrections explained:
1. Checking for NULL before using pointers: The if (!list || !str) check
   is now performed before calling strdup and realloc, which prevents
   null pointer dereferences.
2. Checking the result of strdup: strdup can return NULL if memory
   allocation fails. This also needs to be checked.
3. Checking the result of realloc: If realloc returns NULL, then memory
   has not been allocated and dup must be freed to avoid memory leaks.

Triggers found by static analyzer Svace.

Signed-off-by: Anton Moryakov <ant.v.moryakov@gmail.com>