Fix out-of-bounds panic parsing timestamps (#28)

Found by go-fuzz.

gitdiff/file_header.go

···
 
 	// a valid timestamp can have optional ':' in zone specifier
 	// remove that if it exists so we have a single format
-	if ts[len(ts)-3] == ':' {
+	if len(ts) >= 3 && ts[len(ts)-3] == ':' {
 		ts = ts[:len(ts)-3] + ts[len(ts)-2:]
 	}
 

gitdiff/file_header_test.go

···
 			Input:  "+++ file.txt\t2019-03-21 12:34:56.789 -0700\n",
 			Output: false,
 		},
+		"notTimestamp": {
+			Input:  "+++ file.txt\trandom text\n",
+			Output: false,
+		},
+		"notTimestampShort": {
+			Input:  "+++ file.txt\t0\n",
+			Output: false,
+		},
 	}
 
 	for name, test := range tests {