Avoid extra newline in errors received in FE protocol version 2.

Contrary to what the comment said, the postmaster does in fact end all
its messages in a newline, since server version 7.2. Be tidy and don't
add an extra newline if the error message already has one.

Discussion: https://www.postgresql.org/message-id/CAFBsxsEdgMXc%2BtGfEy9Y41i%3D5pMMjKeH8t8vSAypR3ZnAoQnHg%40mail.gmail.com

src/interfaces/libpq/fe-connect.c

@@ -3277,10 +3277,15 @@ keep_going:						/* We will come back to here until there is
 					conn->inStart = conn->inCursor;
 
 					/*
-					 * The postmaster typically won't end its message with a
-					 * newline, so add one to conform to libpq conventions.
+					 * Before 7.2, the postmaster didn't always end its
+					 * messages with a newline, so add one if needed to
+					 * conform to libpq conventions.
 					 */
-					appendPQExpBufferChar(&conn->errorMessage, '\n');
+					if (conn->errorMessage.len == 0 ||
+						conn->errorMessage.data[conn->errorMessage.len - 1] != '\n')
+					{
+						appendPQExpBufferChar(&conn->errorMessage, '\n');
+					}
 
 					goto error_return;
 				}