fix compiling with NO_HDR NO_LINEAR

stb_image.h

@@ -1032,7 +1032,7 @@ static int stbi__mad3sizes_valid(int a, int b, int c, int add)
 }
 
 // returns 1 if "a*b*c*d + add" has no negative terms/factors and doesn't overflow
-#if !defined(STBI_NO_LINEAR) || !defined(STBI_NO_HDR)
+#if !defined(STBI_NO_LINEAR) || !defined(STBI_NO_HDR) || !defined(STBI_NO_PNM)
 static int stbi__mad4sizes_valid(int a, int b, int c, int d, int add)
 {
    return stbi__mul2sizes_valid(a, b) && stbi__mul2sizes_valid(a*b, c) &&
@@ -1055,7 +1055,7 @@ static void *stbi__malloc_mad3(int a, int b, int c, int add)
    return stbi__malloc(a*b*c + add);
 }
 
-#if !defined(STBI_NO_LINEAR) || !defined(STBI_NO_HDR)
+#if !defined(STBI_NO_LINEAR) || !defined(STBI_NO_HDR) || !defined(STBI_NO_PNM)
 static void *stbi__malloc_mad4(int a, int b, int c, int d, int add)
 {
    if (!stbi__mad4sizes_valid(a, b, c, d, add)) return NULL;