Return 0 for negative "free" and "total" memory reported by the OS

We've had a situation where the MX bean reported negative values for the
free memory of the OS, in those rare cases we want to return a value of
0 rather than blowing up later down the pipeline.

In the event that there is a serialization or creation error with regard
to memory use, this adds asserts so the failure will occur as soon as
possible and give us a better location for investigation.

Resolves elastic#42157

Author: dakrone

server/src/main/java/org/elasticsearch/monitor/os/OsProbe.java

@@ -70,7 +70,12 @@ public long getFreePhysicalMemorySize() {
             return -1;
         }
         try {
-            return (long) getFreePhysicalMemorySize.invoke(osMxBean);
+            final long freeMem = (long) getFreePhysicalMemorySize.invoke(osMxBean);
+            if (freeMem < 0) {
+                logger.warn("OS reported a negative free memory value [{}]", freeMem);
+                return 0;
+            }
+            return freeMem;
         } catch (Exception e) {
             return -1;
         }
@@ -84,7 +89,12 @@ public long getTotalPhysicalMemorySize() {
             return -1;
         }
         try {
-            return (long) getTotalPhysicalMemorySize.invoke(osMxBean);
+            final long totalMem = (long) getTotalPhysicalMemorySize.invoke(osMxBean);
+            if (totalMem < 0) {
+                logger.warn("OS reported a negative total memory value [{}]", totalMem);
+                return 0;
+            }
+            return totalMem;
         } catch (Exception e) {
             return -1;
         }

server/src/main/java/org/elasticsearch/monitor/os/OsStats.java

@@ -228,13 +228,17 @@ public static class Mem implements Writeable, ToXContentFragment {
         private final long free;
 
         public Mem(long total, long free) {
+            assert total >= 0 : "expected total memory to be positive, got: " + total;
+            assert free >= 0 : "expected free memory to be positive, got: " + total;
             this.total = total;
             this.free = free;
         }
 
         public Mem(StreamInput in) throws IOException {
             this.total = in.readLong();
+            assert total >= 0 : "expected total memory to be positive, got: " + total;
             this.free = in.readLong();
+            assert free >= 0 : "expected free memory to be positive, got: " + total;
         }
 
         @Override