Fail cleanly on allocation failures

Previously code in the SWIG tool didn't handle allocation failures
well.  Most places didn't check for NULL return from
malloc()/realloc()/calloc() at all, typically resulting in undefined
behaviour, and some places used assert() to check for a NULL return
(which is a misuse of assert() and such checks disappear if built with
NDEBUG defined leaving us back with undefined behaviour).

All C allocations are now done via wrapper functions (Malloc(),
Realloc() and Calloc()) which emit and error and exit with non-zero
status on failure, so a non-NULL return can be relied upon.

Fixes #1901.

Source/DOH/hash.c

@@ -173,10 +173,7 @@ static void resize(Hash *h) {
     p = p + 2;
   }
 
-  table = (HashNode **) DohMalloc(newsize * sizeof(HashNode *));
-  for (i = 0; i < newsize; i++) {
-    table[i] = 0;
-  }
+  table = (HashNode **) DohCalloc(newsize, sizeof(HashNode *));
 
   /* Walk down the old set of nodes and re-place */
   h->hashsize = newsize;