setup proper error handling for libpng in the png_loader. Now if the PNG

file is corrupted in the middle it won't just print a message and abort
the program.

dlib/image_loader/png_loader.cpp

@@ -87,6 +87,16 @@ namespace dlib
 
 // ----------------------------------------------------------------------------------------
 
+    // Don't do anything when libpng calls us to tell us about an error.  Just return to 
+    // our own code and throw an exception (at the long jump target).
+    void png_loader_user_error_fn_silent(png_structp  png_struct, png_const_charp ) 
+    {
+        longjmp(png_jmpbuf(png_struct),1);
+    }
+    void png_loader_user_warning_fn_silent(png_structp , png_const_charp ) 
+    {
+    }
+
     void png_loader::read_image( const char* filename )
     {
         ld_.reset(new LibpngData);
@@ -110,7 +120,7 @@ namespace dlib
             fclose( fp );
             throw image_load_error(std::string("png_loader: format error in file ") + filename);
         }
-        ld_->png_ptr_ = png_create_read_struct( PNG_LIBPNG_VER_STRING, NULL, NULL, NULL );
+        ld_->png_ptr_ = png_create_read_struct( PNG_LIBPNG_VER_STRING, NULL, &png_loader_user_error_fn_silent, &png_loader_user_warning_fn_silent );
         if ( ld_->png_ptr_ == NULL )
         {
             fclose( fp );
@@ -130,6 +140,16 @@ namespace dlib
             png_destroy_read_struct( &( ld_->png_ptr_ ), &( ld_->info_ptr_ ), ( png_infopp )NULL );
             throw image_load_error(std::string("png_loader: parse error in file ") + filename);
         }
+
+        if (setjmp(png_jmpbuf(ld_->png_ptr_)))
+        {
+            // If we get here, we had a problem writing the file 
+            fclose(fp);
+            png_destroy_read_struct( &( ld_->png_ptr_ ), &( ld_->info_ptr_ ), &( ld_->end_info_ ) );
+            throw image_load_error(std::string("png_loader: parse error in file ") + filename);
+        }
+
+
         png_init_io( ld_->png_ptr_, fp );
         png_set_sig_bytes( ld_->png_ptr_, 8 );
         // flags force one byte per channel output
@@ -149,11 +169,11 @@ namespace dlib
         }
 
         ld_->row_pointers_ = png_get_rows( ld_->png_ptr_, ld_->info_ptr_ );
-        // FIXME: the following call makes libpng crash. Why?
-        //png_read_end( ld_->png_ptr_, ld_->end_info_ );
+
         fclose( fp );
         if ( ld_->row_pointers_ == NULL )
         {
+            png_destroy_read_struct( &( ld_->png_ptr_ ), &( ld_->info_ptr_ ), &( ld_->end_info_ ) );
             throw image_load_error(std::string("png_loader: parse error in file ") + filename);
         }
     }