Fix NPE in DataFormatMatcher#getMatchedFormatName when no match exists (#591)

Change DataFormatMatcher#getMatchedFormatName so it does what the Javadoc says to avoid NPE for non-match; add unit tests to verify.

Author: sleberknight

src/main/java/com/fasterxml/jackson/core/format/DataFormatMatcher.java

@@ -91,7 +91,7 @@ public MatchStrength getMatchStrength() {
      *</pre>
      */
     public String getMatchedFormatName() {
-        return _match.getFormatName();
+        return hasMatch() ? getMatch().getFormatName() : null;
     }
 
     /*

src/test/java/com/fasterxml/jackson/core/format/DataFormatMatcherTest.java

@@ -35,4 +35,24 @@ public void testCreatesDataFormatMatcherTwo() throws IOException {
         verifyException(e, "Illegal start/length");
     }
   }
+
+  public void testGetMatchedFormatNameReturnsNameWhenMatches() {
+    DataFormatMatcher dataFormatMatcher = new DataFormatMatcher(null,
+            new byte[2],
+            1,
+            0,
+            new JsonFactory(),
+            MatchStrength.SOLID_MATCH);
+    assertEquals(JsonFactory.FORMAT_NAME_JSON, dataFormatMatcher.getMatchedFormatName());
+  }
+
+  public void testGetMatchedFormatNameReturnsNullWhenNoMatch() {
+    DataFormatMatcher dataFormatMatcher = new DataFormatMatcher(null,
+            new byte[2],
+            1,
+            0,
+            null,
+            MatchStrength.NO_MATCH);
+    assertNull(dataFormatMatcher.getMatchedFormatName());
+  }
 }