imagehlp: Correctly handle files without NT headers in MapAndLoad.

dlls/imagehlp/access.c

diff --git a/dlls/imagehlp/access.c b/dlls/imagehlp/access.c
index f7f352d..51ec161 100644
--- a/dlls/imagehlp/access.c
+++ b/dlls/imagehlp/access.c
@@ -187,7 +187,12 @@
         goto Error;
     }
 
-    pNtHeader = RtlImageNtHeader(mapping);
+    if (!(pNtHeader = RtlImageNtHeader(mapping)))
+    {
+        WARN("Not an NT header\n");
+        UnmapViewOfFile(mapping);
+        goto Error;
+    }
 
     pLoadedImage->ModuleName       = HeapAlloc(GetProcessHeap(), 0,
                                                strlen(szFileName) + 1);