fix dereference before checking for NULL in the `ImagingTransformAffi…

…ne` function

The `imIn` pointer is checked for `NULL`, but it seems to be dereferenced before this check. You must first make sure that the pointer is not `NULL` before using it.

src/libImaging/Geometry.c

@@ -1035,6 +1035,10 @@ ImagingTransformAffine(
     double xx, yy;
     double xo, yo;
 
+    if (!imOut || !imIn || strcmp(imIn->mode, imOut->mode) != 0) {
+        return (Imaging)ImagingError_ModeError();
+    }    
+
     if (filterid || imIn->type == IMAGING_TYPE_SPECIAL) {
         return ImagingGenericTransform(
             imOut, imIn, x0, y0, x1, y1, affine_transform, a, filterid, fill
@@ -1046,10 +1050,6 @@ ImagingTransformAffine(
         return ImagingScaleAffine(imOut, imIn, x0, y0, x1, y1, a, fill);
     }
 
-    if (!imOut || !imIn || strcmp(imIn->mode, imOut->mode) != 0) {
-        return (Imaging)ImagingError_ModeError();
-    }
-
     if (x0 < 0) {
         x0 = 0;
     }