os: Fix open result check.

Fix defect reported by Coverity Scan.

Argument cannot be negative (NEGATIVE_RETURNS)
negative_returns: f is passed to a parameter that cannot be negative.

CID: 1364709
Fixes: 13fa051356 ("auxiliary/os: add new os_get_command_line() function")
Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Adam Jackson <ajax@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7344>

src/gallium/auxiliary/os/os_process.c

@@ -122,7 +122,7 @@ os_get_command_line(char *cmdline, size_t size)
    }
 #elif defined(PIPE_OS_LINUX)
    int f = open("/proc/self/cmdline", O_RDONLY);
-   if (f) {
+   if (f != -1) {
       const int n = read(f, cmdline, size - 1);
       int i;
       assert(n < size);