OSS-Fuzz: Check img size b4 readers allocate mem

After the completion of the start_input() method, it's too late to check
the image size, because the image readers may have already tried to
allocate memory for the image.  If the width and height are excessively
large, then attempting to allocate memory for the image could slow
performance or lead to out-of-memory errors prior to the fuzz target
checking the image size.

NOTE: Specifically, the aforementioned OOM errors and slow units were
observed with the compression fuzz targets when using MSan.

turbojpeg.c

@@ -2092,22 +2092,17 @@ DLLEXPORT unsigned char *tjLoadImage(const char *filename, int *width,
     THROWG("tjLoadImage(): Unsupported file type");
 
   src->input_file = file;
+#ifdef FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION
+  /* Ignore images larger than 1 Megapixel when fuzzing. */
+  if (flags & TJFLAG_FUZZING)
+    src->max_pixels = 1048576;
+#endif
   (*src->start_input) (cinfo, src);
   (*cinfo->mem->realize_virt_arrays) ((j_common_ptr)cinfo);
 
   *width = cinfo->image_width;  *height = cinfo->image_height;
   *pixelFormat = cs2pf[cinfo->in_color_space];
 
-#ifdef FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION
-  /* Ignore 0-pixel images and images larger than 1 Megapixel when fuzzing.
-     Casting *width to (unsigned long long) prevents integer overflow if
-     (*width) * (*height) > INT_MAX. */
-  if (flags & TJFLAG_FUZZING &&
-      (*width < 1 || *height < 1 ||
-       (unsigned long long)(*width) * (*height) > 1048576))
-    THROWG("tjLoadImage(): Uncompressed image is too large");
-#endif
-
   pitch = PAD((*width) * tjPixelSize[*pixelFormat], align);
   if ((unsigned long long)pitch * (unsigned long long)(*height) >
       (unsigned long long)((size_t)-1) ||