Fixed regex captures handling without PCRE.

Fixed regex captures handling without PCRE.

If PCRE is disabled, captures were treated as normal variables in
ngx_http_script_compile(), while code calculating flushes array length in
ngx_http_compile_complex_value() did not account captures as variables.
This could lead to write outside of the array boundary when setting
last element to -1.

Found with AddressSanitizer.