Ensure methods called by global funcs are init'd

If a hypothetical calling application does something really stupid and
changes cinfo->data_precision after calling jpeg_start_*compress(), then
the precision-specific methods called by jpeg_write_scanlines(),
jpeg_write_raw_data(), jpeg_finish_compress(), jpeg_read_scanlines(),
jpeg_read_raw_data(), or jpeg_start_output() may not be initialized.

Ensure that the first precision-specific method (which will always be
cinfo->main->process_data*(), cinfo->coef->compress_data*(), or
cinfo->coef->decompress_data()) called by any global function that may
be called after jpeg_start_*compress() is initialized and non-NULL.
This increases the likelihood (but does not guarantee) that a
hypothetical stupid calling application will fail gracefully rather than
segfault if it changes cinfo->data_precision after calling
jpeg_start_*compress().  A hypothetical stupid calling application can
still bork itself by changing cinfo->data_precision after initializing
the source manager but before calling jpeg_start_compress(), or after
initializing the destination manager but before calling
jpeg_start_decompress().

jcmainct.c

@@ -6,7 +6,7 @@
  * Lossless JPEG Modifications:
  * Copyright (C) 1999, Ken Murchison.
  * libjpeg-turbo Modifications:
- * Copyright (C) 2022, D. R. Commander.
+ * Copyright (C) 2022, 2024, D. R. Commander.
  * For conditions of distribution and use, see the accompanying README.ijg
  * file.
  *
@@ -146,6 +146,7 @@ _jinit_c_main_controller(j_compress_ptr cinfo, boolean need_full_buffer)
   main_ptr = (my_main_ptr)
     (*cinfo->mem->alloc_small) ((j_common_ptr)cinfo, JPOOL_IMAGE,
                                 sizeof(my_main_controller));
+  memset(main_ptr, 0, sizeof(my_main_controller));
   cinfo->main = (struct jpeg_c_main_controller *)main_ptr;
   main_ptr->pub.start_pass = start_pass_main;