libjavajpeg: Fix compile warning in jchuff.c

gcc warns on s390 and ppcle about negative array overflow at jchuff.c:806.

/priv/d031900/openjdk/jdk-hs/source/src/java.desktop/share/native/libjavajpeg/jchuff.c:808:18: error: array subscript is below array bounds [-Werror=array-bounds]

Adam Farley of IBM provided a fix:

diff --git a/src/java.desktop/share/native/libjavajpeg/jchuff.c b/src/java.desktop/share/native/libjavajpeg/jchuff.c
--- a/src/java.desktop/share/native/libjavajpeg/jchuff.c
+++ b/src/java.desktop/share/native/libjavajpeg/jchuff.c
@@ -805,7 +805,7 @@
   for (i = MAX_CLEN; i > 16; i--) {
     while (bits[i] > 0) {
       j = i - 2; /* find length of new prefix to be used */
- while (bits[j] == 0)
+ while ((bits[j] == 0) && (j > 0))
         j--;

       bits[i] -= 2; /* remove two symbols */