Remove assert when retrieving value of string with invalid chars

Fix for Unity case 1052246.  Debugger was asserting when trying to
retrieve the value of a string that has invalid characters.  The
assert will crash the Unity player/editor.  Changing to detect error,
free the string if necessary, and return an error code to the client.

Author: Brian Raderman

mono/mini/debugger-agent.c

@@ -11456,7 +11456,13 @@ string_commands (int command, guint8 *p, guint8 *end, Buffer *buf)
 		} else {
 			MonoError error;
 			s = mono_string_to_utf8_checked (str, &error);
-			mono_error_assert_ok (&error);
+			if (!mono_error_ok (&error)) {
+				if (s)
+					g_free (s);
+
+				return ERR_INVALID_ARGUMENT;
+			}
+
 			buffer_add_string (buf, s);
 			g_free (s);
 		}