ImBuf: replace exit(1) with assert when writing IRIS images

In this case memory past the buffer bounds would have been written,
potentially crashing. Replace the check with an assert since this
should never happen.

Also don't call exit as failing to write a file shouldn't exit Blender.

source/blender/imbuf/intern/iris.c

@@ -854,10 +854,9 @@ static bool output_iris(const char *filepath,
           len = compressrow((const uchar *)zptr, rlebuf, CHANOFFSET(z - 4), xsize);
         }
       }
-      if (len > rlebuflen) {
-        fprintf(stderr, "output_iris: rlebuf is too small - bad poop\n");
-        exit(1);
-      }
+
+      BLI_assert_msg(len <= rlebuflen, "The length calculated for 'rlebuflen' was too small!");
+
       goodwrite *= fwrite(rlebuf, len, 1, outf);
       starttab[y + z * ysize] = pos;
       lengthtab[y + z * ysize] = len;