[ASan] return 0 for current allocated bytes if malloc/free are never happend (llvm#67394)

This is found during address sanitizer enablement on AIX.

On platforms that has no malloc/free calls before user's malloc/free
calls, `__sanitizer_get_current_allocated_bytes()` should return 0.
Otherwise the case like
`compiler-rt/test/sanitizer_common/TestCases/allocator_interface.cpp`
will fail at below scenario:
```
void Test(int size) { 
  auto allocated_bytes_before = __sanitizer_get_current_allocated_bytes();
  int *p = (int *)malloc(size);

  assert(__sanitizer_get_current_allocated_bytes() >=
         size + allocated_bytes_before);   // if allocated_bytes_before is 1, this assert will fail. allocated_bytes_before should be 0
}
```

Author: Chen Zheng

compiler-rt/lib/asan/asan_stats.cpp

@@ -142,7 +142,7 @@ uptr __sanitizer_get_current_allocated_bytes() {
   uptr freed = stats.freed;
   // Return sane value if malloced < freed due to racy
   // way we update accumulated stats.
-  return (malloced > freed) ? malloced - freed : 1;
+  return (malloced > freed) ? malloced - freed : 0;
 }
 
 uptr __sanitizer_get_heap_size() {
@@ -161,7 +161,7 @@ uptr __sanitizer_get_free_bytes() {
                   + stats.malloced_redzones;
   // Return sane value if total_free < total_used due to racy
   // way we update accumulated stats.
-  return (total_free > total_used) ? total_free - total_used : 1;
+  return (total_free > total_used) ? total_free - total_used : 0;
 }
 
 uptr __sanitizer_get_unmapped_bytes() {

compiler-rt/test/asan/TestCases/Posix/current_allocated_bytes.cpp

@@ -17,7 +17,7 @@ void* allocate(void *arg) {
 }
 
 void* check_stats(void *arg) {
-  assert(__sanitizer_get_current_allocated_bytes() > 0);
+  assert(__sanitizer_get_current_allocated_bytes() >= 0);
   return 0;
 }