AArch64: String.indexOf may incorrectly handle empty strings.

On AArch64, the String.indexOf() may produce incorrect results for empty
strings due to absence of the argument length check in
MacroAssembler::string_indexof_char().

Following test demonstrates the problem:

public class Test1 {
  public static void main(String ... str) {

    System.out.println("Begin");

    for (int i = 0 ; i < 100000; i ++) {
      String emptyString = "";
      for(int c=0; c<0xFFFF; c++) {
        int dot = emptyString.indexOf((char)c, -1);
        if (dot != -1) {
          System.out.println("indexOf returned index " + dot);
        }
      }
    }

    System.out.println("End");
  }
}

Run:
java -XX:-CompactStrings Test1


Suggested fix:

diff --git a/src/hotspot/cpu/aarch64/macroAssembler_aarch64.cpp
b/src/hotspot/cpu/aarch64/macroAssembler_aarch64.cpp
--- a/src/hotspot/cpu/aarch64/macroAssembler_aarch64.cpp
+++ b/src/hotspot/cpu/aarch64/macroAssembler_aarch64.cpp
@@ -4860,6 +4860,8 @@
   Register ch1 = rscratch1;
   Register result_tmp = rscratch2;
 
+ cbz(cnt1, NOMATCH);
+
   cmp(cnt1, (u1)4);
   br(LT, DO1_SHORT);