server: Allow 0-write length in WriteProcessMemory().

dlls/kernel32/tests/virtual.c

@@ -114,6 +114,14 @@ static void test_VirtualAllocEx(void)
     ok(b && (bytes_read == alloc_size), "%Iu bytes read\n", bytes_read);
     ok(!memcmp(src, dst, alloc_size), "Data from remote process differs\n");
 
+    /* test 0 length */
+    bytes_written = 0xdeadbeef;
+    b = WriteProcessMemory(hProcess, addr1, src, 0, &bytes_written);
+    ok((b && !bytes_written) || broken(!b && GetLastError() == ERROR_INVALID_PARAMETER), "write failed: %lu\n", GetLastError());
+    bytes_read = 0xdeadbeef;
+    b = ReadProcessMemory(hProcess, addr1, src, 0, &bytes_read);
+    ok(b && !bytes_read, "read failed: %lu\n", GetLastError());
+
     /* test invalid source buffers */
 
     b = VirtualProtect( src + 0x2000, 0x2000, PAGE_NOACCESS, &old_prot );

server/process.c

@@ -1659,7 +1659,6 @@ DECL_HANDLER(write_process_memory)
     {
         data_size_t len = get_req_data_size();
         if (len) write_process_memory( process, req->addr, len, get_req_data() );
-        else set_error( STATUS_INVALID_PARAMETER );
         release_object( process );
     }
 }