Work around valgrind/MSan SIMD false positives

Referring to
https://sourceforge.net/p/libjpeg-turbo/bugs/48,
https://sourceforge.net/p/libjpeg-turbo/bugs/82,
 #15, #238, #253, and #619,
valgrind and MSan have failed to properly detect data initialization by
libjpeg-turbo's x86 SIMD extensions for the entire 14 years that
libjpeg-turbo has been a project, resulting in false positives unless
libjpeg-turbo is built with WITH_SIMD=0 or run with JSIMD_FORCENONE=1.
This commit introduces a new C preprocessor macro (ZERO_BUFFERS) that,
if set, causes libjpeg-turbo to zero certain buffers in order to work
around the specific valgrind/MSan test failures caused by the
aforementioned false positives.  This allows us to more closely
approximate the production configuration of libjpeg-turbo when testing
with valgrind or MSan.

Closes #781

jcphuff.c

@@ -650,6 +650,11 @@ encode_mcu_AC_first(j_compress_ptr cinfo, JBLOCKROW *MCU_data)
   size_t bits[8 / SIZEOF_SIZE_T];
   int max_coef_bits = cinfo->data_precision + 2;
 
+#ifdef ZERO_BUFFERS
+  memset(values_unaligned, 0, sizeof(values_unaligned));
+  memset(bits, 0, sizeof(bits));
+#endif
+
   entropy->next_output_byte = cinfo->dest->next_output_byte;
   entropy->free_in_buffer = cinfo->dest->free_in_buffer;
 
@@ -915,6 +920,11 @@ encode_mcu_AC_refine(j_compress_ptr cinfo, JBLOCKROW *MCU_data)
   size_t zerobits, signbits;
   size_t bits[16 / SIZEOF_SIZE_T];
 
+#ifdef ZERO_BUFFERS
+  memset(absvalues_unaligned, 0, sizeof(absvalues_unaligned));
+  memset(bits, 0, sizeof(bits));
+#endif
+
   entropy->next_output_byte = cinfo->dest->next_output_byte;
   entropy->free_in_buffer = cinfo->dest->free_in_buffer;