histogram: avoid dereferencing null profile pointer.

Also add a on-display log information about the profile being
replaced by Rec2020.

Fixes #15370.

src/libs/histogram.c

@@ -775,9 +775,15 @@ static void _lib_histogram_process_vectorscope
   if(!vs_prof || !dt_is_valid_colormatrix(vs_prof->matrix_in[0][0]))
   {
     dt_print(DT_DEBUG_ALWAYS,
-             "[histogram] unsupported vectorscope profile %i %s, it will be replaced with linear Rec2020\n",
-             vs_prof->type, vs_prof->filename);
-    vs_prof = dt_ioppr_add_profile_info_to_list(darktable.develop, DT_COLORSPACE_LIN_REC2020, "", DT_INTENT_RELATIVE_COLORIMETRIC);
+             "[histogram] unsupported vectorscope profile %i %s,"
+             " it will be replaced with linear Rec2020\n",
+             vs_prof ? vs_prof->type     : 0,
+             vs_prof ? vs_prof->filename : "unsupported");
+    dt_control_log(_("unsupported vectorscope profile selected,"
+                     " it will be replaced with linear Rec2020"));
+    vs_prof = dt_ioppr_add_profile_info_to_list
+      (darktable.develop,
+       DT_COLORSPACE_LIN_REC2020, "", DT_INTENT_RELATIVE_COLORIMETRIC);
   }
 
   _lib_histogram_vectorscope_bkgd(d, vs_prof);