Merge pull request #943 from Unity-Technologies/unity-trunk-avoid-crash-on-invalid-method-header

Return NULL so client can raise exception rather than asserting on invalid method header (case 1032601)

mono/metadata/loader.c

@@ -2356,7 +2356,9 @@ mono_method_get_header (MonoMethod *method)
 		return NULL;
 
 	loc = mono_image_rva_map (img, rva);
-	g_assert (loc);
+	if (!loc) {
+		return NULL;
+	}
 
 	header = mono_metadata_parse_mh_full (img, mono_method_get_generic_container (method), loc);