maketables: avoid misleading values in case flipping table (#313)

The tables generated by pcre2_maketables() include one that maps
all lowercase characters on the first 255 code points to their
corresponding upper case code point, but fails to notice that
toupper() could return a larger code point and therefore result
in the store of a truncated and unrelated code instead.

Restrict all values to what is valid for uint8_t and document
in the test case the failure for character 'μ'[1] (U+00B5) and
that was incorrectly getting back 924 (U+039C) from macOS fr_FR,
and resulting in an incorrect case equivalent with the truncated
value of 159.

[1] https://en.wikipedia.org/wiki/Mu_(letter)

src/pcre2_maketables.c

@@ -52,8 +52,6 @@ PCRE2_DFTABLES is defined. */
 #  include "pcre2_internal.h"
 #endif
 
-
-
 /*************************************************
 *           Create PCRE2 character tables        *
 *************************************************/
@@ -98,7 +96,11 @@ for (i = 0; i < 256; i++) *p++ = tolower(i);
 
 /* Next the case-flipping table */
 
-for (i = 0; i < 256; i++) *p++ = islower(i)? toupper(i) : tolower(i);
+for (i = 0; i < 256; i++)
+  {
+  int c = islower(i)? toupper(i) : tolower(i);
+  *p++ = (c < 256)? c : i;
+  }
 
 /* Then the character class tables. Don't try to be clever and save effort on
 exclusive ones - in some locales things may be different.

testdata/testinput3

@@ -75,6 +75,11 @@
 \= Expect no match
     �cole
 
+/\xb5/i
+    �
+\= Expect no match
+    \x9c
+
 /\W+/
     >>>\xaa<<<
     >>>\xba<<< 

testdata/testoutput3

@@ -108,6 +108,13 @@ Subject length lower bound = 1
     �cole
 No match
 
+/\xb5/i
+    �
+ 0: �
+\= Expect no match
+    \x9c
+No match
+
 /\W+/
     >>>\xaa<<<
  0: >>>

testdata/testoutput3A

@@ -108,6 +108,13 @@ Subject length lower bound = 1
     �cole
 No match
 
+/\xb5/i
+    �
+ 0: �
+\= Expect no match
+    \x9c
+No match
+
 /\W+/
     >>>\xaa<<<
  0: >>>

testdata/testoutput3B

@@ -108,6 +108,13 @@ Subject length lower bound = 1
     �cole
 No match
 
+/\xb5/i
+    �
+ 0: �
+\= Expect no match
+    \x9c
+No match
+
 /\W+/
     >>>\xaa<<<
  0: >>>