Set MMapFile invalid when mmap failed.

Print more info when failed to open EXR.

test_tinyexr.cc

@@ -13,8 +13,8 @@
 #include <vector>
 
 // Uncomment if you want to use system provided zlib.
-// #define TINYEXR_USE_MINIZ (0)
-// #include <zlib.h>
+//#define TINYEXR_USE_MINIZ (0)
+//#include <zlib.h>
 
 #define TINYEXR_IMPLEMENTATION
 #include "tinyexr.h"
@@ -216,7 +216,7 @@ int test_main(int argc, char** argv) {
 
   int ret = IsEXR(input_filename);
   if (ret != TINYEXR_SUCCESS) {
-    fprintf(stderr, "Header err. code %d\n", ret);
+    fprintf(stderr, "File not found or given file is not a EXR format. code %d\n", ret);
     exit(-1);
   }
 
@@ -230,6 +230,7 @@ int test_main(int argc, char** argv) {
     FreeEXRErrorMessage(err);
     return ret;
   }
+
   // SaveAsPFM("output.pfm", width, height, image);
   ret = SaveEXR(image, width, height, 4 /* =RGBA*/,
                 1 /* = save as fp16 format */, "output.exr", &err);

tinyexr.h

@@ -6737,6 +6737,7 @@ struct MemoryMappedFile {
     data = reinterpret_cast<unsigned char *>(
         mmap(0, size, PROT_READ, MAP_SHARED, posix_descriptor, 0));
     if (data == MAP_FAILED) {
+      data = nullptr;
       return;
     }
 #else
@@ -6761,11 +6762,13 @@ struct MemoryMappedFile {
     size = static_cast<size_t>(ftell_result);
     if (fseek(fp, 0, SEEK_SET) != 0) {
       fclose(fp);
+      size = 0;
       return;
     }
 
     data = reinterpret_cast<unsigned char *>(malloc(size));
     if (!data) {
+      size = 0;
       fclose(fp);
       return;
     }