Minidump without context crashes the processor (#153). r=bryner

http://groups.google.com/group/google-breakpad-dev/browse_thread/thread/2d221bd478bec550


git-svn-id: http://google-breakpad.googlecode.com/svn/trunk@148 4c0a9323-5329-0410-9bdc-e9ce6186880e

src/processor/stackwalker.cc

@@ -139,6 +139,9 @@ Stackwalker* Stackwalker::StackwalkerForCPU(
     const CodeModules *modules,
     SymbolSupplier *supplier,
     SourceLineResolverInterface *resolver) {
+  if (!context)
+    return NULL;
+
   Stackwalker *cpu_stackwalker = NULL;
 
   u_int32_t cpu = context->GetContextCPU();