PNM loader: Fix reading PNM bitmaps.

Attempting to read a PNM bitmap (ASCII format) would cause a lockup due
to infinite loop, and in certain cases write access outside allocated
memory.

Fixes CVE-2016-6348 (out-of-bounds writes ... presumably - CVE text not
disclosed yet).
Found by Neelima Krishnan, Intel Corporation.

src/modules/loaders/loader_pnm.c

@@ -179,6 +179,7 @@ load(ImlibImage * im, ImlibProgressFunction progress,
                                goto quit_error;
                             ptr2++;
                             i++;
+                            x++;
                          }
                     }
                   if (progress &&