byte-extract: switch to pcre_copy_substring

Fixes memory leaks (str_ptr was never freed).
Fixes cppcheck warning:
    [src/detect-byte-extract.c:268]: (error) Uninitialized variable: str_ptr

Bug #1046.
byte-test: switch to pcre_copy_substring

Fixes cppcheck:
[src/detect-bytejump.c:360]: (error) Uninitialized variable: str_ptr
[src/detect-bytejump.c:372]: (error) Uninitialized variable: str_ptr
[src/detect-bytejump.c:375]: (error) Uninitialized variable: str_ptr

Bug #1046.