oss-fuzz 43680: Cope with out-of-memory error in stack extension

The minimal PS-like interpreter in pdfi allows the operand stand to be extended,
but the code was missing handling of the memory allocation failing for the
extended stack.

pdf/pdf_fontps.h

@@ -1,4 +1,4 @@
-/* Copyright (C) 2020-2021 Artifex Software, Inc.
+/* Copyright (C) 2020-2022 Artifex Software, Inc.
    All Rights Reserved.
 
    This software is provided AS-IS with no warranty, either express or
@@ -201,6 +201,9 @@ static inline int pdf_ps_stack_push(pdf_ps_ctx_t *s)
                 s->cur = s->stack + currsize - 1;
                 s->toplim = s->stack + newsize - PDF_PS_STACK_GROW_SIZE;
             }
+            else {
+                return_error(gs_error_VMerror);
+            }
         }
     }
     s->cur++;