RFR: 8291956: Simplify the loop condition in sun.net.httpserver.Request constructor

[thyecust]

The condition at line 57 (after while) will evaluate to false
if startLine == null, so the previous if-condition is covered.

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Commit messages:
 - simplify loop contidition
 - Merge branch 'openjdk:master' into if
 - remove redundant if-condition after readLine()

Changes: https://git.openjdk.org/jdk/pull/9571/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9571&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8291956
  Stats: 4 lines in 1 file changed: 0 ins; 3 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/9571.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9571/head:pull/9571

PR: https://git.openjdk.org/jdk/pull/9571