Return 0 when detecting the amount of memory fails

Fixes bug 14802;  bugfix on 0.2.5.4-alpha.

changes/bug14802

@@ -0,0 +1,4 @@
+  o Minor bugfix:
+    - Make an educated guess about how much memory the system has when our
+      detection functionality fails on some operating system.  Fixes bug 14802;
+      bugfix on 0.2.5.4-alpha.

src/common/compat.c

@@ -3517,7 +3517,7 @@ get_total_system_memory_impl(void)
   size_t len = sizeof(memsize);
   int mib[2] = {CTL_HW, HW_USERMEM};
   if (sysctl(mib,2,&memsize,&len,NULL,0))
-    return -1;
+    return 0;
 
   return memsize;