pcre2_compile: report read_number overflow errors when needed

Backreferences starting with 8 or 9 and larger than INT_MAX
where being mishandled.

Instead of ignoring overflow errors, report them upstream so
they can be handled.

Reported-by: Nicholas Wilson

src/pcre2_compile.c

@@ -1906,15 +1906,15 @@ else
       \1 to \9 are always back references. \8x and \9x are too; \1x to \7x
       are octal escapes if there are not that many previous captures. */
 
-      if (read_number(&ptr, ptrend, -1, INT_MAX/10 - 1, 0, &s, errorcodeptr) &&
+      if (read_number(&ptr, ptrend, -1, INT_MAX/10 - 1, ERR61, &s, errorcodeptr) &&
           (s < 10 || oldptr[-1] >= CHAR_8 || s <= (int)cb->bracount))
         {
         if (s > (int)MAX_GROUP_NUMBER) *errorcodeptr = ERR61;
           else escape = -s;     /* Indicates a back reference */
         break;
         }
 
-      ptr = oldptr;      /* Put the pointer back and fall through */
+      if (c <= CHAR_7) ptr = oldptr; /* Put the pointer back for fall through */
       }
 
     /* Handle a digit following \ when the number is not a back reference, or
@@ -1924,6 +1924,12 @@ else
 
     if (c >= CHAR_8) break;
 
+    /* read_number() could have returned an overflow error, but that is no longer
+    relevant since we are about to read the number again, but this time as an
+    octal. */
+
+    *errorcodeptr = 0;
+
     /* Fall through */
 
     /* \0 always starts an octal number, but we may drop through to here with a

testdata/testinput2

@@ -4520,6 +4520,12 @@
 
 /(?(1)()\983040\2)/
 
+/a\800000b/
+
+/a\800000000b/
+
+/a\8000000000b/
+
 /(*LIMIT_MATCH=)abc/
 
 /(*CRLF)(*LIMIT_MATCH=)abc/

testdata/testoutput2

@@ -14718,6 +14718,15 @@ No match
 /(?(1)()\983040\2)/
 Failed: error 161 at offset 14: subpattern number is too big
 
+/a\800000b/
+Failed: error 161 at offset 8: subpattern number is too big
+
+/a\800000000b/
+Failed: error 161 at offset 11: subpattern number is too big
+
+/a\8000000000b/
+Failed: error 161 at offset 11: subpattern number is too big
+
 /(*LIMIT_MATCH=)abc/
 Failed: error 160 at offset 14: (*VERB) not recognized or malformed