Fix crash in wxImage::Scale()

Caused by arithmetic overflow of a signed addition and subsequent
right-shift of that (now negative) value and use as an array index.
Could only realistically occur on platforms where long is 32 bits.
See #18550

src/common/image.cpp

@@ -526,21 +526,21 @@ wxImage wxImage::ResampleNearest(int width, int height) const
         }
     }
 
-    long old_height = M_IMGDATA->m_height,
+    const unsigned long old_height = M_IMGDATA->m_height;
-         old_width  = M_IMGDATA->m_width;
+    const unsigned long old_width  = M_IMGDATA->m_width;
-    long x_delta = (old_width<<16) / width;
+    const unsigned long x_delta = (old_width  << 16) / width;
-    long y_delta = (old_height<<16) / height;
+    const unsigned long y_delta = (old_height << 16) / height;
 
     unsigned char* dest_pixel = target_data;
 
-    long y = 0;
+    unsigned long y = 0;
-    for ( long j = 0; j < height; j++ )
+    for (int j = 0; j < height; j++)
     {
         const unsigned char* src_line = &source_data[(y>>16)*old_width*3];
         const unsigned char* src_alpha_line = source_alpha ? &source_alpha[(y>>16)*old_width] : 0 ;
 
-        long x = 0;
+        unsigned long x = 0;
-        for ( long i = 0; i < width; i++ )
+        for (int i = 0; i < width; i++)
         {
             const unsigned char* src_pixel = &src_line[(x>>16)*3];
             const unsigned char* src_alpha_pixel = source_alpha ? &src_alpha_line[(x>>16)] : 0 ;