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/memory.c

@@ -14,6 +14,9 @@
 
 #include "dohint.h"
 
+#include <stdio.h>
+#include <stdlib.h>
+
 #ifndef DOH_POOL_SIZE
 #define DOH_POOL_SIZE         4194304
 #endif
@@ -48,10 +51,7 @@ static int pools_initialized = 0;
 static void CreatePool() {
   Pool *p = 0;
   p = (Pool *) DohMalloc(sizeof(Pool));
-  assert(p);
-  p->ptr = (DohBase *) DohMalloc(sizeof(DohBase) * PoolSize);
-  assert(p->ptr);
-  memset(p->ptr, 0, sizeof(DohBase) * PoolSize);
+  p->ptr = (DohBase *) DohCalloc(PoolSize, sizeof(DohBase));
   p->len = PoolSize;
   p->blen = PoolSize * sizeof(DohBase);
   p->current = 0;
@@ -234,3 +234,40 @@ void DohMemoryDebug(void) {
 #endif
 
 }
+
+static void allocation_failed(size_t n, size_t size) {
+  /* Report and exit as directly as possible to try to avoid further issues due
+   * to lack of memory. */
+  if (n == 1) {
+#if defined __STDC_VERSION__ && __STDC_VERSION__-0 >= 19901L
+    fprintf(stderr, "Failed to allocate %zu bytes\n", size);
+#else
+    fprintf(stderr, "Failed to allocate %lu bytes\n", (unsigned long)size);
+#endif
+  } else {
+#if defined __STDC_VERSION__ && __STDC_VERSION__-0 >= 19901L
+    fprintf(stderr, "Failed to allocate %zu*%zu bytes\n", n, size);
+#else
+    fprintf(stderr, "Failed to allocate %lu*%lu bytes\n", (unsigned long)n, (unsigned long)size);
+#endif
+  }
+  exit(EXIT_FAILURE);
+}
+
+void *DohMalloc(size_t size) {
+  void *p = malloc(size);
+  if (!p) allocation_failed(1, size);
+  return p;
+}
+
+void *DohRealloc(void *ptr, size_t size) {
+  void *p = realloc(ptr, size);
+  if (!p) allocation_failed(1, size);
+  return p;
+}
+
+void *DohCalloc(size_t n, size_t size) {
+  void *p = calloc(n, size);
+  if (!p) allocation_failed(n, size);
+  return p;
+}