BUG/MEDIUM: http: don't store url_decode() result in the samples's length

BUG/MEDIUM: http: don't store url_decode() result in the samples's length

By convenience or laziness we used to store url_decode()'s return code
into smp->data.u.str.data. The result checks applied there compare it
to 0 while it's now unsigned since commit 843b7cb ("MEDIUM: chunks: make
the chunk struct's fields match the buffer struct "). Let's clean this up
and test the result itself without storing it first.

No backport is needed.