Revert "unix,win: handle zero-sized allocations uniformly"

This reverts commit ba78023.
Inconsistently working is better than consistently broken
(until the issues this creates can be fixed with libuv).
Refs: libuv#2076

(cherry picked from commit 7273f6c)
(cherry picked from commit 3071fe1)

src/uv-common.c

@@ -73,9 +73,7 @@ char* uv__strndup(const char* s, size_t n) {
 }
 
 void* uv__malloc(size_t size) {
-  if (size > 0)
-    return uv__allocator.local_malloc(size);
-  return NULL;
+  return uv__allocator.local_malloc(size);
 }
 
 void uv__free(void* ptr) {
@@ -94,10 +92,7 @@ void* uv__calloc(size_t count, size_t size) {
 }
 
 void* uv__realloc(void* ptr, size_t size) {
-  if (size > 0)
-    return uv__allocator.local_realloc(ptr, size);
-  uv__free(ptr);
-  return NULL;
+  return uv__allocator.local_realloc(ptr, size);
 }
 
 void* uv__reallocf(void* ptr, size_t size) {