detect/content: account for distance variables

detect/content: account for distance variables

Under some cases (below), the depth and offset values are used
twice. This commit disregards the distance variable (if any), when
computing the final depth.

These rules are logically equivalent::
1. alert tcp any any -> any 8080 (msg:"distance name"; flow:to_server; content:"Authorization:"; content:"5f71ycy"; distance:0; byte_extract:1,0,option_len,string,relative; content:!"|38|"; distance:option_len; within:1; content:"|37|"; distance:-1; within:1; content:"|49|"; distance:option_len; within:1; sid:1;)
2. alert tcp any any -> any 8080 (msg:"distance number"; flow:to_server; content:"Authorization:"; content:"5f71ycy"; distance:0; byte_extract:1,0,option_len,string,relative; content:!"|38|"; distance:7; within:1; content:"|37|"; distance:-1; within:1; content:"|49|"; distance:option_len; within:1; sid:2;)

The differences:
Rule 1: content:!"|38|"; distance:option_len; within:1; //option_len == 7

Rule 2: content:!"|38|"; distance:7; within:1;

Without this commit, rule 2 triggers an alert but rule 1 doesn't.

Issue: 7390