Add check to see if stack pointer is off the stack according to the memory

src/processor/exploitability_linux.h

Index: src/processor/exploitability_linux.h
===================================================================
--- src/processor/exploitability_linux.h	(revision 1481)
+++ src/processor/exploitability_linux.h	(working copy)
@@ -58,6 +58,10 @@
   // This method checks the exception that triggered the creation of the
   // minidump and reports whether the exception suggests no exploitability.
   bool BenignCrashTrigger(const MDRawExceptionStream *raw_exception_stream);
+
+  // Checks if the stack pointer points to a memory mapping that is not
+  // labelled as the stack.
+  bool StackPointerOffStack(uint64_t stack_ptr);
 };
 
 }  // namespace google_breakpad