fix: avoid unchecked nullptr deref on invalid lcms transform

On invalid data the requested transform from lcms may return nun
successfully. Hence we need to check the return value and early
the function to avoid a segfault.
To distinguish the concrete error, a new error code (45) has been
introduced.

Signed-off-by: Levente Polyak <levente@leventepolyak.net>

rust/ffi.rs

@@ -31,6 +31,7 @@ pub enum pngquant_error {
     LIBPNG_FATAL_ERROR = 25,
     WRONG_INPUT_COLOR_TYPE = 26,
     LIBPNG_INIT_ERROR = 35,
+    LCMS_FATAL_ERROR = 45,
     TOO_LARGE_FILE = 98,
     TOO_LOW_QUALITY = 99,
 }

rwpng.c

@@ -376,6 +376,12 @@ static pngquant_error rwpng_read_image24_libpng(FILE *infile, png24_image *mainp
                                                       hOutProfile, TYPE_RGBA_8,
                                                       INTENT_PERCEPTUAL,
                                                       omp_get_max_threads() > 1 ? cmsFLAGS_NOCACHE : 0);
+        if(!hTransform) {
+            png_destroy_read_struct(&png_ptr, &info_ptr, NULL);
+            cmsCloseProfile(hOutProfile);
+            cmsCloseProfile(hInProfile);
+            return LCMS_FATAL_ERROR;
+        }
 
         #pragma omp parallel for \
             if (mainprog_ptr->height*mainprog_ptr->width > 8000) \

rwpng.h

@@ -32,6 +32,7 @@ typedef enum {
     LIBPNG_FATAL_ERROR = 25,
     WRONG_INPUT_COLOR_TYPE = 26,
     LIBPNG_INIT_ERROR = 35,
+    LCMS_FATAL_ERROR = 45,
     TOO_LARGE_FILE = 98,
     TOO_LOW_QUALITY = 99,
 } pngquant_error;