timing.c: avoid referencing garbage value

Found with Clang's `scan-build` tool.

When get_timer() is called with `reset` set to 1, the value of
t->start.tv_sec is used as a rvalue without being initialized first.
This is relatively harmless because the result of get_timer() is not
used by the callers when called in "reset mode". However, scan-build
prints a warning.

Silence the warning by only calculating the delta on non-reset runs,
returning zero otherwise.

library/timing.c

@@ -283,15 +283,16 @@ unsigned long get_timer( struct hr_time *val, int reset )
 
     gettimeofday( &offset, NULL );
 
-    delta = ( offset.tv_sec  - t->start.tv_sec  ) * 1000
-          + ( offset.tv_usec - t->start.tv_usec ) / 1000;
-
     if( reset )
     {
         t->start.tv_sec  = offset.tv_sec;
         t->start.tv_usec = offset.tv_usec;
+        return( 0 );
     }
 
+    delta = ( offset.tv_sec  - t->start.tv_sec  ) * 1000
+          + ( offset.tv_usec - t->start.tv_usec ) / 1000;
+
     return( delta );
 }