raster-interpreter.c: Fix crash in 'scan_ps()' found by fuzzer

Fuzzer using `_cupsRasterExecPS()` found a way how to pass NULL into
`scan_ps()`, causing crash - we have to sanitize the argument for NULL
to fix it.

Fixes #831

CHANGES.md

@@ -8,6 +8,7 @@ Changes in CUPS v2.4.8 (TBA)
   successfully printed jobs (Issue #830)
 - Added warning if the device has to be asked for 'all,media-col-database' separately
   (Issue #829)
+- Fixed crash in `scan_ps()` if incoming argument is NULL (Issue #831)
 - Fixed memory leak when creating color profiles (Issue #815)
 - Fixed memory leak when unloading a job (Issue #813)
 - Raised `cups_enum_dests()` timeout for listing available IPP printers (Issue #751)

cups/raster-interpret.c

@@ -1048,6 +1048,8 @@ scan_ps(_cups_ps_stack_t *st,		/* I  - Stack */
   int			parens;		/* Parenthesis nesting level */
 
 
+  if (!*ptr)
+    return (NULL);
  /*
   * Skip leading whitespace...
   */