Check input parameters in _TIFFMergeFields()

Fixes #624

libtiff/tif_dirinfo.c

@@ -574,9 +574,20 @@ int _TIFFMergeFields(TIFF *tif, const TIFFField info[], uint32_t n)
 {
     static const char module[] = "_TIFFMergeFields";
     static const char reason[] = "for fields array";
-    /* TIFFField** tp; */
     uint32_t i;
 
+    /* Check input */
+    if (tif == NULL || info == NULL)
+    {
+        TIFFWarningExtR(tif, module, "An input pointer is NULL");
+        return 0;
+    }
+    else if (n == 0)
+    {
+        TIFFWarningExtR(tif, module, "Number of fields to be merged is zero");
+        return 0;
+    }
+
     tif->tif_foundfield = NULL;
 
     if (tif->tif_fields && tif->tif_nfields > 0)
@@ -596,7 +607,6 @@ int _TIFFMergeFields(TIFF *tif, const TIFFField info[], uint32_t n)
         return 0;
     }
 
-    /* tp = tif->tif_fields + tif->tif_nfields; */
     for (i = 0; i < n; i++)
     {
         const TIFFField *fip = TIFFFindField(tif, info[i].field_tag, TIFF_ANY);