cmd/guru: fix nil deref when position is not in Go source

+ test

Fixes issue golang/go#14684

Change-Id: I56023bf36c307d02c71b4ddf08aee9c229fc66c1
Reviewed-on: https://go-review.googlesource.com/20247
Reviewed-by: Daniel Morsing <daniel.morsing@gmail.com>
diff --git a/cmd/guru/testdata/src/README.txt b/cmd/guru/testdata/src/README.txt
new file mode 100644
index 0000000..34fc41a
--- /dev/null
+++ b/cmd/guru/testdata/src/README.txt
@@ -0,0 +1,2 @@
+This is not a Go source file.
+Used by TestIssue14684.