Return a small allocation from realloc(ptr, 0). (#460)

An annoying amount of real-world code (e.g. mandoc, BSD sort) treats a
NULL return from `realloc` as a failure, even when requesting a size of
0.  This code is wrong (the standard explicitly permits a return of NULL
from realloc when given a size 0) but working around it in snmalloc is
easier than fixing it everywhere.

src/test/func/malloc/malloc.cc

@@ -36,12 +36,19 @@ void check_result(size_t size, size_t align, void* p, int err, bool null)
     failed = true;
   }
   const auto alloc_size = our_malloc_usable_size(p);
-  const auto expected_size = round_size(size);
+  auto expected_size = round_size(size);
 #ifdef SNMALLOC_PASS_THROUGH
   // Calling system allocator may allocate a larger block than
   // snmalloc. Note, we have called the system allocator with
   // the size snmalloc would allocate, so it won't be smaller.
   const auto exact_size = false;
+  // We allocate MIN_ALLOC_SIZE byte for 0-sized allocations (and so round_size
+  // will tell us that the minimum size is MIN_ALLOC_SIZE), but the system
+  // allocator may return a 0-sized allocation.
+  if (size == 0)
+  {
+    expected_size = 0;
+  }
 #else
   const auto exact_size = align == 1;
 #endif