Fix overflow bug in new /u code. Fixes ClusterFuzz 13073.

src/pcre2_compile.c

@@ -1596,6 +1596,7 @@ else
       {
       uint32_t xc;
       
+      if (ptr >= ptrend) break;
       if (*ptr == CHAR_LEFT_CURLY_BRACKET && 
           (extra_options & PCRE2_EXTRA_ALT_BSUX) != 0)
         {

testdata/testinput5

@@ -817,6 +817,9 @@
 /^\u{0000000000010ffff}/utf,extra_alt_bsux
     \x{10ffff}
 
+/\u/utf,alt_bsux
+    \\u
+
 /^a+[a\x{200}]/B,utf
     aa
 

testdata/testoutput5

@@ -1738,6 +1738,10 @@ Failed: error 173 at offset 6: disallowed Unicode code point (>= 0xd800 && <= 0x
     \x{10ffff}
  0: \x{10ffff}
 
+/\u/utf,alt_bsux
+    \\u
+ 0: u
+
 /^a+[a\x{200}]/B,utf
 ------------------------------------------------------------------
         Bra