qtestcase: Fix buffer over-run, '\0' appended beyond buffer end

Noticed by Coverity (CID 161673).  If the file being read contains
enough to fill the buffer, read() shall do that and return the nbytes
it was passed; as this was the size of the buffer, subsequently
writing a '\0' at this index in buffer is out of bounds.  Fortunately,
/proc/self/status is typically < 1k so fits well inside the 2k buffer.
All the same, we can safely pass sizeof(buffer) - 1 as nbytes and *be
sure* of not getting a buffer over-run.

Change-Id: Ib620a330fbc94f0579c953737f7c4417ca449968
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@theqtcompany.com>

src/testlib/qtestcase.cpp

@@ -2514,7 +2514,7 @@ static bool debuggerPresent()
     if (fd == -1)
         return false;
     char buffer[2048];
-    ssize_t size = read(fd, buffer, sizeof(buffer));
+    ssize_t size = read(fd, buffer, sizeof(buffer) - 1);
     if (size == -1) {
         close(fd);
         return false;