MemoryBuffer: don't force mmap when stat fails

Fix error handling introduced in r127426 that could result in MemoryBuffers not
having null termination.

llvm-svn: 208396
diff --git a/llvm/lib/Support/MemoryBuffer.cpp b/llvm/lib/Support/MemoryBuffer.cpp
index f95383d..26c3a2e 100644
--- a/llvm/lib/Support/MemoryBuffer.cpp
+++ b/llvm/lib/Support/MemoryBuffer.cpp
@@ -320,9 +320,8 @@
   // RequiresNullTerminator = false and MapSize != -1.
   if (FileSize == size_t(-1)) {
     sys::fs::file_status Status;
-    error_code EC = sys::fs::status(FD, Status);
-    if (EC)
-      return EC;
+    if (sys::fs::status(FD, Status))
+      return false;
     FileSize = Status.getSize();
   }