Fix panics for invalid utf-8 json streams (#119)

`stext_page_as_json_from_page` was using `unwrap` for reading the mupdf
json stream. This was causing panic when the stream was not a valid
utf-8.

The function now returns the `error` in such cases.

Author: pratyushmittal

src/page.rs

@@ -252,7 +252,7 @@ impl Page {
         }?;
         let mut buf = unsafe { Buffer::from_raw(inner) };
         let mut res = String::new();
-        buf.read_to_string(&mut res).unwrap();
+        buf.read_to_string(&mut res)?;
         Ok(res)
     }
 

tests/files/no-json.pdf

@@ -117,3 +117,11 @@ fn test_issue_i32_box() {
         assert!(stext_page.is_ok());
     }
 }
+
+#[test]
+fn test_issue_no_json() {
+    let doc = PdfDocument::open("tests/files/no-json.pdf").unwrap();
+    let page = doc.load_page(0).unwrap();
+    let json = page.stext_page_as_json_from_page(1.0);
+    assert!(json.is_err());
+}