BUG/MEDIUM: map: don't store exp_replace() result in the trash's length

BUG/MEDIUM: map: don't store exp_replace() result in the trash's length

By convenience or laziness we used to store exp_replace()'s return code
into str->data. The result checks applied there compare str->data to -1
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.