(perl #132245) don't try to process a char range with no preceding char

A range like \N{}-0 eventually results in compilation failing, but
before that, get_and_check_backslash_N_name() attempts to treat
the memory before the empty output of \N{} as a character.

t/lib/croak/toke

@@ -413,3 +413,9 @@ EXPECT
 Illegal operator following parameter in a subroutine signature at - line 3, near "($a += 1"
 syntax error at - line 3, near "($a += 1"
 Execution of - aborted due to compilation errors.
+########
+# NAME tr/// range with empty \N{} at the start
+tr//\N{}-0/;
+EXPECT
+Unknown charname '' at - line 1, within string
+Execution of - aborted due to compilation errors.

toke.c

@@ -2969,9 +2969,9 @@ S_scan_const(pTHX_ char *start)
 
                 /* Here, we don't think we're in a range.  If the new character
                  * is not a hyphen; or if it is a hyphen, but it's too close to
-                 * either edge to indicate a range, then it's a regular
-                 * character. */
-                if (*s != '-' || s >= send - 1 || s == start) {
+                 * either edge to indicate a range, or if we haven't output any
+                 * characters yet then it's a regular character. */
+                if (*s != '-' || s >= send - 1 || s == start || d == SvPVX(sv)) {
 
                     /* A regular character.  Process like any other, but first
                      * clear any flags */