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.

cjpeg.c

@@ -690,6 +690,9 @@ main(int argc, char **argv)
   /* Figure out the input file format, and set up to read it. */
   src_mgr = select_file_type(&cinfo, input_file);
   src_mgr->input_file = input_file;
+#ifdef CJPEG_FUZZER
+  src_mgr->max_pixels = 1048576;
+#endif
 
   /* Read the input file header to obtain file size & colorspace. */
   (*src_mgr->start_input) (&cinfo, src_mgr);
@@ -709,9 +712,7 @@ main(int argc, char **argv)
     jpeg_stdio_dest(&cinfo, output_file);
 
 #ifdef CJPEG_FUZZER
-  if (cinfo.image_width < 1 || cinfo.image_height < 1 ||
-      (unsigned long long)cinfo.image_width * cinfo.image_height > 1048576 ||
-      setjmp(myerr.setjmp_buffer))
+  if (setjmp(myerr.setjmp_buffer))
     HANDLE_ERROR()
 #endif