Various macros assume the argument is not negative

This cuts the ASAN failures in half. From 42 to 20 on my macOS server.

Macros like `isaletter()` assume the argument is not negative. But
there is at least one place where it is called with a value of -1 from
fcget(). Specifically, in `sh_lex()`:

    case S_DOT: {
        // Make sure next character is alpha.
        n = fcgetc();
        if (n > 0) {
            if (n == '.') n = fcgetc();
            if (n > 0) fcseek(-LEN);
        }
        if (isaletter(n) || n == LBRACT) continue;

In the rksh.sh unit test `n` is -1 thus causing a reference to the element
before one of the lexer state tables.

Harden those macros against that possibility so they don't use some
random data.

Related #971

Author: krader1961

src/cmd/ksh93/include/lexstates.h

@@ -91,13 +91,13 @@
 
 #undef LEN
 #define LEN _Fcin.fclen
-#define isaname(c) ((c) > 0x7f ? isalpha(c) : sh_lexstates[ST_NAME][(c)] == S_NOP)
-#define isaletter(c) ((c) > 0x7f ? isalpha(c) : sh_lexstates[ST_DOL][(c)] == S_ALP && (c) != '.')
 #define STATE(s, c) (s[mbwide() ? ((c = fcmbget(&LEN)), LEN > 1 ? 'a' : c) : (c = fcget())])
-#define isadigit(c) (sh_lexstates[ST_DOL][c] == S_DIG)
+#define isaname(c) ((c) < 0 ? 0 : ((c) > 0x7f ? isalpha(c) : sh_lexstates[ST_NAME][(c)] == S_NOP))
+#define isaletter(c) ((c) < 0 ? 0 : ((c) > 0x7f ? isalpha(c) : sh_lexstates[ST_DOL][(c)] == S_ALP && (c) != '.'))
+#define isadigit(c) ((c) < 0 ? 0 : sh_lexstates[ST_DOL][c] == S_DIG)
 #define isastchar(c) ((c) == '@' || (c) == '*')
-#define isexp(c) (sh_lexstates[ST_MACRO][c] == S_PAT || (c) == '$' || (c) == '`')
-#define ismeta(c) (sh_lexstates[ST_NAME][c] == S_BREAK)
+#define isexp(c) ((c) < 0 ? 0 : (sh_lexstates[ST_MACRO][c] == S_PAT || (c) == '$' || (c) == '`'))
+#define ismeta(c) ((c) < 0 ? 0 : sh_lexstates[ST_NAME][c] == S_BREAK)
 
 extern const char *sh_lexstates[ST_NONE];
 extern const char e_lexversion[];