peering: fix compilation warning

> verify.c:58:30: warning: comparison of array 'vp->strvalue' equal to a null pointer is always false [-Wtautological-pointer-compare]
>     if (vp->lvalue==0 || vp->strvalue==0)
>                          ~~~~^~~~~~~~  ~
> 1 warning generated.

(cherry picked from commit bb545d0)

modules/peering/verify.c

@@ -55,7 +55,7 @@ static inline int extract_avp(VALUE_PAIR* vp, unsigned short *flags,
     char *end;
 
     /* empty? */
-    if (vp->lvalue==0 || vp->strvalue==0)
+    if (vp->lvalue==0)
         goto error;
 
     p = vp->strvalue;