Handle undefined shifts in CPDF_PSEngine.

core/fpdfapi/page/fpdf_page_func.cpp

Index: core/fpdfapi/page/fpdf_page_func.cpp
diff --git a/core/fpdfapi/page/fpdf_page_func.cpp b/core/fpdfapi/page/fpdf_page_func.cpp
index d2e08a511c6566e583e9f7e29c0221058580ed1c..248a13ab84038a712a171d05e9ac07c8b70a6b70 100644
--- a/core/fpdfapi/page/fpdf_page_func.cpp
+++ b/core/fpdfapi/page/fpdf_page_func.cpp
@@ -343,11 +343,12 @@ FX_BOOL CPDF_PSEngine::DoOperator(PDF_PSOP op) {
     case PSOP_BITSHIFT: {
       int shift = (int)Pop();
       int i = (int)Pop();
-      if (shift > 0) {
+      if (shift < -31 || shift > 31)

[Tom Sepez, 2016/10/12 20:17:58]

http://en.cppreference.com/w/cpp/language/operator_arithmetic

Plucking out the C++11 sections, 

For signed and positive a, the value of a << b is a * 2b if it is representable
the return type, otherwise the behavior is undefined.	
For negative a, the behavior of a << b is undefined.

So there are still cases where this might be flagged.  It's pretty much
impossible to check this properly :( ...

The only way to do this, it would seem, is to use int64_t 
(which is how the safe_int packages handles this?)

+        Push(0);
+      else if (shift >= 0)
         Push(i << shift);
-      } else {
+      else
         Push(i >> -shift);
-      }
       break;
     }
     case PSOP_TRUE: