[osx] don't crash debugallocator on 0-sized allocations

OSX's malloc zones facility is calling 'usable size' before each (!)
free and uses size to choose between our allocator and something
else. And returning 0 breaks this choice. Which happens if we got
0-sized malloc request, which our tests exercise couple times. So we
don't let debug allocator on OSX to have 0 sized chunks. This unbreaks
debug allocation tests on OSX.

src/debugallocation.cc

@@ -1034,6 +1034,15 @@ void __malloctrace_write(const char *buf, size_t size) {
 // General debug allocation/deallocation
 
 static inline void* DebugAllocate(size_t size, int type) {
+#if defined(__APPLE__)
+  // OSX malloc zones integration has some odd behavior. When
+  // GetAllocatedSize returns 0 it appears to assume something wrong
+  // about the pointer. And since in debug allocator we can return 0
+  // if original size was also 0, lets avoid this case. But only on
+  // OSX. It weakens debug checks a bit, but it unbreaks some tests
+  // (around realloc/free of 0-sized chunks).
+  if (size == 0) size = 1;
+#endif
   MallocBlock* ptr = MallocBlock::Allocate(size, type);
   if (ptr == NULL)  return NULL;
   MALLOC_TRACE("malloc", size, ptr->data_addr());

src/tests/debugallocation_test.cc

@@ -264,6 +264,9 @@ TEST(DebugAllocationTest, GetAllocatedSizeTest) {
   // exactly requested size, since debug_allocation doesn't allow users
   // to write more than that.
   for (int i = 0; i < 10; ++i) {
+#ifdef __APPLE__
+    if (i == 0) continue;
+#endif
     void *p = noopt(malloc(i));
     EXPECT_EQ(i, MallocExtension::instance()->GetAllocatedSize(p));
     free(p);