S_parse_ident: Restructure croaks

This removes the test for the identifier being too long at the beginning
of the loop.

Almost all the branches already have their own checks for this, and by
restructuring those slightly, this one becomes extraneous, except for
the one branch that didn't have a check.  And it turns out this was
wrong for that branch, which is the one that turns an apostrophe package
separator into ::.  The removed code assumed that the output doesn't
expand, whereas each single apostrophe is in fact replaced by two
characters.  This commit corrects that.

And it consolidates the handling to the target of a goto at the end of
the function.  Right now, there is no real advantage in that, but a
future commit will make the handling more complicated, so a single point
for it will be useful.

Author: khwilliamson

toke.c

@@ -10579,8 +10579,6 @@ S_parse_ident(pTHX_ const char *s, const char * const s_end,
     const bool check_dollar = flags & CHECK_DOLLAR;
 
     while (s < s_end) {
-        if (*d >= e)
-            croak("%s", ident_too_long);
 
         /* For non-UTF8, variables that match ASCII \w are a superset of
          * variables that start with IDFIRST, so we have to look at the
@@ -10600,8 +10598,9 @@ S_parse_ident(pTHX_ const char *s, const char * const s_end,
 
             /* Here we have found the end of the identifier */
             Size_t this_length = t - s;
-            if (*d + this_length > e)
-                croak("%s", ident_too_long);
+            if (*d + this_length >= e) {
+                goto too_long;
+            }
 
             /* And copy the whole thing in one operation */
             Copy(s, *d, this_length, char);
@@ -10614,13 +10613,21 @@ S_parse_ident(pTHX_ const char *s, const char * const s_end,
              * digit */
             do {
                 *(*d)++ = *s++;
-            } while (isWORDCHAR_A(*s) && *d < e);
+
+                if (*d >= e) {
+                    goto too_long;
+                }
+            } while (isWORDCHAR_A(*s));
         }
         else if (   allow_package
                  && *s == '\''
                  && FEATURE_APOS_AS_NAME_SEP_IS_ENABLED
                  && isIDFIRST_lazy_if_safe(s + 1, s_end, is_utf8))
         {   /* Convert the apostrophe to "::" */
+            if (*d >= e - 2) {
+                goto too_long;
+            }
+
             *(*d)++ = ':';
             *(*d)++ = ':';
             s++;
@@ -10632,6 +10639,10 @@ S_parse_ident(pTHX_ const char *s, const char * const s_end,
             */
            && !(check_dollar && s[2] == '$'))
         {
+            if (*d >= e - 2) {
+                goto too_long;
+            }
+
             *(*d)++ = *s++;
             *(*d)++ = *s++;
         }
@@ -10646,6 +10657,9 @@ S_parse_ident(pTHX_ const char *s, const char * const s_end,
      * function declares it as const so as to indicate that it doesn't change
      * it, and it can be called using a const parameter */
     return (char *) s;
+
+  too_long:
+    croak("%s", ident_too_long);
 }
 
 char *