[perl #72590] Panic if 'use strict' and forget 'my'

That bug happens when we detect a compilation error in the statement
being parsed, and when the continuation of the parsing of that same
statement needs to load the file unicore/Name.pl via charnames.pm.
In that case perl gets confused, fails to parse Name.pl because
the parser is already in error, and also fails to properly rewind
to a normal error-reporting state.

This patch does not attempt to fix the whole error-reporting process;
instead, it simply prevents perl from trying to load charnames if it has
already recorded a parse error. So, in a way, it hides the bug under
the carpet. However, this is a safe fix, suitable for a code-freeze
stage.

regcomp.c

@@ -6722,6 +6722,9 @@ S_reg_namedseq(pTHX_ RExC_state_t *pRExC_state, UV *valuep, I32 *flagp)
         if ( he_str ) {
             sv_str = HeVAL(he_str);
             cached = 1;
+	} else if (PL_parser && PL_parser->error_count > 0) {
+	    /* Don't attempt to load charnames if we're already in error */
+	    vFAIL("Too many errors, cannot continue parsing");
         } else {
             dSP ;