Fixed NULL pointer dereference, thanks Ozkan Sezer

src/video/SDL_blit_N.c

@@ -125,11 +125,7 @@ calc_swizzle32(const SDL_PixelFormat * srcfmt, const SDL_PixelFormat * dstfmt)
                                                        0x0C);
     vector unsigned char vswiz;
     vector unsigned int srcvec;
-#define RESHIFT(X) (3 - ((X) >> 3))
-    Uint32 rmask = RESHIFT(srcfmt->Rshift) << (dstfmt->Rshift);
-    Uint32 gmask = RESHIFT(srcfmt->Gshift) << (dstfmt->Gshift);
-    Uint32 bmask = RESHIFT(srcfmt->Bshift) << (dstfmt->Bshift);
-    Uint32 amask;
+    Uint32 rmask, gmask, bmask, amask;
 
     if (!srcfmt) {
         srcfmt = &default_pixel_format;
@@ -138,6 +134,11 @@ calc_swizzle32(const SDL_PixelFormat * srcfmt, const SDL_PixelFormat * dstfmt)
         dstfmt = &default_pixel_format;
     }
 
+#define RESHIFT(X) (3 - ((X) >> 3))
+    rmask = RESHIFT(srcfmt->Rshift) << (dstfmt->Rshift);
+    gmask = RESHIFT(srcfmt->Gshift) << (dstfmt->Gshift);
+    bmask = RESHIFT(srcfmt->Bshift) << (dstfmt->Bshift);
+
     /* Use zero for alpha if either surface doesn't have alpha */
     if (dstfmt->Amask) {
         amask =
@@ -149,6 +150,7 @@ calc_swizzle32(const SDL_PixelFormat * srcfmt, const SDL_PixelFormat * dstfmt)
                           0xFFFFFFFF);
     }
 #undef RESHIFT
+
     ((unsigned int *) (char *) &srcvec)[0] = (rmask | gmask | bmask | amask);
     vswiz = vec_add(plus, (vector unsigned char) vec_splat(srcvec, 0));
     return (vswiz);