FIX: getting rid of "shifting a negative signed value is undefined" warnings using Clang

Author: Oldes

src/core/u-jpg.c

@@ -3740,10 +3740,10 @@ static const int extend_test[16] =   /* entry n is 2**(n-1) */
     0x0100, 0x0200, 0x0400, 0x0800, 0x1000, 0x2000, 0x4000 };
 
 static const int extend_offset[16] = /* entry n is (-1 << n) + 1 */
-  { 0, ((-1)<<1) + 1, ((-1)<<2) + 1, ((-1)<<3) + 1, ((-1)<<4) + 1,
-    ((-1)<<5) + 1, ((-1)<<6) + 1, ((-1)<<7) + 1, ((-1)<<8) + 1,
-    ((-1)<<9) + 1, ((-1)<<10) + 1, ((-1)<<11) + 1, ((-1)<<12) + 1,
-    ((-1)<<13) + 1, ((-1)<<14) + 1, ((-1)<<15) + 1 };
+  { 0, -(1<<1) + 1, -(1<<2) + 1, -(1<<3) + 1, -(1<<4) + 1,
+    -(1<<5) + 1, -(1<<6) + 1, -(1<<7) + 1, -(1<<8) + 1,
+    -(1<<9) + 1, -(1<<10) + 1, -(1<<11) + 1, -(1<<12) + 1,
+    -(1<<13) + 1, -(1<<14) + 1, -(1<<15) + 1 };
 
 #endif /* AVOID_TABLES */
 
@@ -10339,10 +10339,10 @@ static const int p_extend_test[16] =   /* entry n is 2**(n-1) */
     0x0100, 0x0200, 0x0400, 0x0800, 0x1000, 0x2000, 0x4000 };
 
 static const int p_extend_offset[16] = /* entry n is (-1 << n) + 1 */
-  { 0, ((-1)<<1) + 1, ((-1)<<2) + 1, ((-1)<<3) + 1, ((-1)<<4) + 1,
-    ((-1)<<5) + 1, ((-1)<<6) + 1, ((-1)<<7) + 1, ((-1)<<8) + 1,
-    ((-1)<<9) + 1, ((-1)<<10) + 1, ((-1)<<11) + 1, ((-1)<<12) + 1,
-    ((-1)<<13) + 1, ((-1)<<14) + 1, ((-1)<<15) + 1 };
+  { 0, -(1<<1) + 1, -(1<<2) + 1, -(1<<3) + 1, -(1<<4) + 1,
+    -(1<<5) + 1, -(1<<6) + 1, -(1<<7) + 1, -(1<<8) + 1,
+    -(1<<9) + 1, -(1<<10) + 1, -(1<<11) + 1, -(1<<12) + 1,
+    -(1<<13) + 1, -(1<<14) + 1, -(1<<15) + 1 };
 
 #endif /* AVOID_TABLES */