PATCH: [perl #12406] regnode panic

This was due to an "=" that should have been a "+=", which ended up in
this situation not allocating enough space for the pattern.  This is not
likely to cause real field problems since the generated pattern won't
succeed, and the problem is warned about and would be corrected.

regcomp.c

@@ -10940,7 +10940,7 @@ S_regpiece(pTHX_ RExC_state_t *pRExC_state, I32 *flagp, U32 depth)
                      * enough space for all the things we are about to throw
                      * away, but we can shrink it by the ammount we are about
                      * to re-use here */
-                    RExC_size = PREVOPER(RExC_size) - regarglen[(U8)OPFAIL];
+                    RExC_size += PREVOPER(RExC_size) - regarglen[(U8)OPFAIL];
                 }
                 else {
                     ckWARNreg(RExC_parse, "Quantifier {n,m} with n > m can't match");

t/re/pat.t

@@ -23,7 +23,7 @@ BEGIN {
     skip_all_without_unicode_tables();
 }
 
-plan tests => 775;  # Update this when adding/deleting tests.
+plan tests => 776;  # Update this when adding/deleting tests.
 
 run_tests() unless caller;
 
@@ -1711,6 +1711,16 @@ EOP
 		like($error, qr{Reference to nonexistent group},
 				'gave appropriate error for qr{()(?1)}n');
 	}
+
+	{
+            # [perl #126406] panic with unmatchable quantifier
+            my $code='
+                no warnings "regexp";
+                "" =~ m/(.0\N{6,0}0\N{6,0}000000000000000000000000000000000)/;
+            ';
+            fresh_perl_is($code, "", {},
+                            "perl [#126406] panic");
+	}
 } # End of sub run_tests
 
 1;