Minor refactoring based on static analysis on the code of images: (#2891)

- Replace deprecated string.h with cstring.
- Switch to nullptr.

torchvision/csrc/cpu/image/read_image_cpu.cpp

 #include "read_image_cpu.h"
-#include <string.h>
+#include <cstring>
 
 torch::Tensor decode_image(const torch::Tensor& data) {
   // Check that the input tensor dtype is uint8

torchvision/csrc/cpu/image/read_write_file_cpu.cpp

@@ -46,7 +46,7 @@ void write_file(std::string filename, torch::Tensor& data) {
   auto fileCStr = filename.c_str();
   FILE* outfile = fopen(fileCStr, "wb");
 
-  TORCH_CHECK(outfile != NULL, "Error opening output file");
+  TORCH_CHECK(outfile != nullptr, "Error opening output file");
 
   fwrite(fileBytes, sizeof(uint8_t), data.numel(), outfile);
   fclose(outfile);