Bug 188976: avoid calculating the difference of two unsigned integer if

the difference is really a negative number.  Thanks to Rick Swift for the
patch.

nsprpub/lib/libc/src/strstr.c

@@ -57,13 +57,15 @@ PL_strrstr(const char *big, const char *little)
 {
     const char *p;
     PRUint32 ll;
+    PRUint32 bl;
 
     if( ((const char *)0 == big) || ((const char *)0 == little) ) return (char *)0;
     if( ((char)0 == *big) || ((char)0 == *little) ) return (char *)0;
 
     ll = PL_strlen(little);
-    p = &big[ PL_strlen(big) - ll ];
-    if( p < big ) return (char *)0;
+    bl = PL_strlen(big);
+    if( bl < ll ) return (char *)0;
+    p = &big[ bl - ll ];
 
     for( ; p >= big; p-- )
         if( *little == *p )