Storable: protect from empty retrieve_vstring

empty sv detected by coverity

Author: rurban

dist/Storable/Storable.xs

@@ -4543,7 +4543,7 @@ static SV *retrieve_blessed(pTHX_ stcxt_t *cxt, const char *cname)
      * Retrieve object and bless it.
      */
 
-    sv = retrieve(aTHX_ cxt, classname);	/* First SV which is SEEN will be blessed */
+    sv = retrieve(aTHX_ cxt, classname); /* First SV which is SEEN will be blessed */
     if (malloced_classname)
         Safefree(malloced_classname);
 
@@ -5537,9 +5537,9 @@ static SV *retrieve_vstring(pTHX_ stcxt_t *cxt, const char *cname)
     TRACEME(("retrieve_vstring (#%d), len = %d", (int)cxt->tagnum, len));
 
     READ(s, len);
-
     sv = retrieve(aTHX_ cxt, cname);
-
+    if (!sv)
+        return (SV *) 0;		/* Failed */
     sv_magic(sv,NULL,PERL_MAGIC_vstring,s,len);
     /* 5.10.0 and earlier seem to need this */
     SvRMAGICAL_on(sv);
@@ -5572,7 +5572,8 @@ static SV *retrieve_lvstring(pTHX_ stcxt_t *cxt, const char *cname)
     SAFEPVREAD(s, len, s);
 
     sv = retrieve(aTHX_ cxt, cname);
-
+    if (!sv)
+        return (SV *) 0;		/* Failed */
     sv_magic(sv,NULL,PERL_MAGIC_vstring,s,len);
     /* 5.10.0 and earlier seem to need this */
     SvRMAGICAL_on(sv);
@@ -6445,7 +6446,7 @@ static SV *old_retrieve_array(pTHX_ stcxt_t *cxt, const char *cname)
         if (c != SX_ITEM)
             (void) retrieve_other(aTHX_ cxt, 0);/* Will croak out */
         TRACEME(("(#%d) item", (int)i));
-        sv = retrieve(aTHX_ cxt, 0);						/* Retrieve item */
+        sv = retrieve(aTHX_ cxt, 0);		/* Retrieve item */
         if (!sv)
             return (SV *) 0;
         if (av_store(av, i, sv) == 0)