QBenchmarkPerfEvents: help Coverity understand the code

Coverity thinks that r may be negative and not -1 and warns that the
cast to quint64 may overflow.

Change the check for `== -1` to `< 0` to help Coverity (and other
readers of the code) understand that this cannot happen.

Amends c63420a117fe67107466d806890e901d091cb1d5.

Pick-to: 6.9 6.8 6.5 6.2 5.15
Coverity-Id: 474172
Change-Id: I7615e0ef152e1e6e9cbc6dec1b8ecd4f8879521c
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/testlib/qbenchmarkperfevents.cpp

@@ -546,7 +546,7 @@ static quint64 rawReadValue(int fd)
     while (nread < sizeof results) {
         char *ptr = reinterpret_cast<char *>(&results);
         qint64 r = qt_safe_read(fd, ptr + nread, sizeof results - nread);
-        if (r == -1) {
+        if (r < 0) {
             perror("QBenchmarkPerfEventsMeasurer::readValue: reading the results");
             exit(1);
         }