avcodec/xpmdec: avoid "magic" numbers in function hex_char_to_number()

Signed-off-by: Paul B Mahol <onemda@gmail.com>

libavcodec/xpmdec.c

@@ -188,9 +188,9 @@ static const ColorEntry color_table[] = {
 static unsigned hex_char_to_number(uint8_t x)
 {
     if (x >= 'a' && x <= 'f')
-        x -= 87;
+        x -= 'a' - 10;
     else if (x >= 'A' && x <= 'F')
-        x -= 55;
+        x -= 'A' - 10;
     else if (x >= '0' && x <= '9')
         x -= '0';
     else