Fix strnlen_P/strlen_P off-by-4 error

Random crashes, often on String constructors using a PSTR, would occur due
to the accelerated strnlen_P going past the end of the string. Would make
debug builds fail, too (ESP.getVersionString() failure).

Fix to fall through to normal copy on a word that's got a 0 byte anywhere
in it.

Author: earlephilhower