Add check to clang 6.0 compiler bug workaround

Extending on b68222e4e75d6cbe23c533f140733248df046c7e to ensure that it still
has the exact same behaviour as the previous code, as discussed with @hpvb.

(cherry picked from commit 03e11c13bfb1649fc9cf9a460249bce5e7f07462)

servers/physics/collision_solver_sat.cpp

@@ -346,7 +346,9 @@ public:
 
 		//use the smallest depth
 
-		min_B = -min_B;
+		if (min_B < 0.0) { // could be +0.0, we don't want it to become -0.0
+			min_B = -min_B;
+		}
 
 		if (max_B < min_B) {
 			if (max_B < best_depth) {