in string::search(), don't call memchr() if ptr == NULL.

noticed when tbl started logging lots of assertion errors generated from my
local copy of libc which has (a modified) assert() tests throughout.

gnu/usr.bin/groff/libgroff/string.cc

@@ -270,7 +270,7 @@ void string::clear()
 
 int string::search(char c) const
 {
-  char *p = (char *)memchr(ptr, c, len);
+  char *p = ptr ? (char *)memchr(ptr, c, len) : NULL;
   return p ? p - ptr : -1;
 }