internal/driver: fixes for fetch.go

[egonelbre]

url.Parse can parse a Windows absolute path without problems. This can cause mapping file to be cleared, even though it's not a remote source.

For #710

[codecov-commenter]

Codecov Report

Merging #711 (91569a6) into main (c488b8f) will increase coverage by 0.01%. The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #711      +/-   ##
==========================================
+ Coverage   63.70%   63.72%   +0.01%     
==========================================
  Files          41       41              
  Lines        6489     6492       +3     
==========================================
+ Hits         4134     4137       +3     
  Misses       1913     1913              
  Partials      442      442              

Impacted Files	Coverage Δ
...c/github.com/google/pprof/internal/driver/fetch.go	63.26% <0.00%> (+0.37%)	:arrow_up:

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