Fix assertions for glfwSetGamma value

The NaN assert was implicit in the other ones.  The lower bound assert
incorrectly allowed a value of zero.

Related to #1387.
diff --git a/src/monitor.c b/src/monitor.c
index f7de550..0be3d20 100644
--- a/src/monitor.c
+++ b/src/monitor.c
@@ -431,8 +431,7 @@
     unsigned short values[256];
     GLFWgammaramp ramp;
     assert(handle != NULL);
-    assert(gamma == gamma);
-    assert(gamma >= 0.f);
+    assert(gamma > 0.f);
     assert(gamma <= FLT_MAX);
 
     _GLFW_REQUIRE_INIT();