isl_*alloc*: return NULL if ctx argument is NULL

isl_*alloc*: return NULL if ctx argument is NULL

Whenever an isl function needs to perform an allocation, it calls
one of the isl_*alloc* functions on an isl_ctx obtained either from
the user or from a call to one of the *_get_ctx functions.
If this isl_ctx pointer is NULL, this means that either
the original isl_ctx_alloc failed or that the pointer on which
the *_get_ctx function was called was NULL.
In both cases, we are in an error state and we should not
allocate any more memory, but instead return as soon as possible.
We therefore return NULL from the isl_*alloc* functions,
an error condition which should already be handled by the caller.

Signed-off-by: Sven Verdoolaege <skimo@kotnet.org>