Replace g_malloc{0,} with g_try_malloc{0,}.

The g_malloc()/g_malloc0() versions exit/segfault if not enough memory
is available, which is not a good thing in libsigrok.

Instead, we use the g_try_malloc()/g_try_malloc0() variants, which
return NULL if not enough memory is available, so that the caller can
handle the error properly.