Fix strlen_P and strcpy_P edge cases

Ran the included test suite on ESP8266 tstring.c with the following defines:
 #define MAX_1 50
 #define memcmp memcmp_P
 #define memcpy memcpy_P
 #define memmem memmem_P
 #define memchr memchr_P
 #define strcat strcat_P
 #define strncat strncat_P
 #define strcpy strcpy_P
 #define strlen strlen_P
 #define strnlen strnlen_P
 #define strcmp strcmp_P
 #define strncmp strncmp_P

Uncovered edge case and return value problems in the optimized versions of
the strnlen_P and strncpy_P functions.  Corrected.

Author: earlephilhower