Fix compilation on GCC 4.9.0

Probably not the greatest fix but somehow either count needs to be signed, or length unsigned.

Fixes the following:

/home/jraspass/ngx_mod_psgi/src/ngx_http_psgi_input_stream.c: In function ‘PerlIONginxInput_read’:
/home/jraspass/ngx_mod_psgi/src/ngx_http_psgi_input_stream.c:53:15: error: comparison between signed and unsigned integer expressions [-Werror=sign-compare]
     len = len > count ? count : len;
               ^
/home/jraspass/ngx_mod_psgi/src/ngx_http_psgi_input_stream.c:53:31: error: signed and unsigned type in conditional expression [-Werror=sign-compare]
     len = len > count ? count : len;