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jbig2dec: Limit scanline index in MMR coded data.

According to the T.6 specification the pixel references, e.g. a0,
may point to any of the pixels (indices 0 through N-1) on a
scanline, one pixel to the left of a scanline (index MINUS1) or
one pixel beyond the scanline (index N). These indicies are all
positive (despite its name MINUS1 is defined to UINT32_MAX).

This commit changes the index type from signed to unsigned to
reflect this. Moreover, when looking for changing elements,
make sure to limit the returned index to N.

--- a/jbig2_mmr.c

+++ b/jbig2_mmr.c

@@ -735,20 +735,22 @@

 #define getbit(buf, x) ( ( buf[x >> 3] >> ( 7 - (x & 7) ) ) & 1 )

-static int

+static uint32_t

 jbig2_find_changing_element(const byte *line, uint32_t x, uint32_t w)

 {

     int a, b;

-    if (line == 0)

-        return (int)w;

+    if (line == NULL)

+        return w;

     if (x == MINUS1) {

         a = 0;

         x = 0;

-    } else {

+    } else if (x < w) {

         a = getbit(line, x);

         x++;

+    } else {

+        return x;

     }

     while (x < w) {

@@ -761,10 +763,10 @@

     return x;

 }

-static int

+static uint32_t

 jbig2_find_changing_element_of_color(const byte *line, uint32_t x, uint32_t w, int color)

 {

-    if (line == 0)

+    if (line == NULL)

         return w;

     x = jbig2_find_changing_element(line, x, w);

     if (x < w && getbit(line, x) != color)