jchuff.c/flush_bits(): Guard against put_bits < 0

This fixes a UBSan negative shift warning, reported by OSS-Fuzz, that
occurred when attempting to transform a specially-crafted malformed
arithmetic-coded JPEG image into a baseline Huffman-coded JPEG
destination image with default Huffman tables.  This issue probably
had a similar root cause to the issue fixed in
31a301389b, but in this case, the issue
only occurred with the SIMD baseline Huffman encoder in libjpeg-turbo
2.1.x.  It was not reproducible in 2.0.x or 3.0.x or when using the
C baseline Huffman encoder.

jchuff.c

@@ -518,7 +518,7 @@ flush_bits(working_state *state)
     temp = (JOCTET)(put_buffer >> put_bits);
     EMIT_BYTE(temp)
   }
-  if (put_bits) {
+  if (put_bits > 0) {
     /* fill partial byte with ones */
     temp = (JOCTET)((put_buffer << (8 - put_bits)) | (0xFF >> put_bits));
     EMIT_BYTE(temp)