Fix annoying silent crash in builtins.sh under ASan

Under ASan the builtins.sh script silently exits with exit status 1
and no backtrace. After isolating the bug into a simplified script,
I managed to reproduce it with an actual stacktrace. Reproducer:
   set -o xtrace  # Shows how far the script goes before crashing
                  # (which is inconsistent and system-dependent
                  # AFAICT; ALL_LIBCMD can affect it).
   bltin='/opt/ast/bin/basename
   /opt/ast/bin/cat
   /opt/ast/bin/cp
   /opt/ast/bin/cut
   /opt/ast/bin/dirname
   /opt/ast/bin/getconf
   /opt/ast/bin/ln
   /opt/ast/bin/mktemp
   /opt/ast/bin/mv'  # Feel free to expand this with other commands
                     # from libcmd if you with.
   for i in ${bltin}
   do ({ PATH=/opt/ast/bin; "${bltin##*/}" --this-option-does-not-exist; } 2>&1)
   done
Stacktrace obtained after much effort:
=================================================================
==116622==ERROR: AddressSanitizer: heap-use-after-free on address 0x502000000e50 at pc 0x72ad1bb52b7f bp 0x7ffc8b5a0cd0 sp 0x7ffc8b5a0478
READ of size 3 at 0x502000000e50 thread T0
    #0 0x72ad1bb52b7e in memcpy /usr/src/debug/gcc/gcc/libsanitizer/sanitizer_common/sanitizer_common_interceptors_memintrinsics.inc:115
    ksh93#1 0x5bb0a922f3de in synthesize /home/johno/GitRepos/KornShell/ksh/src/lib/libast/port/astconf.c:466
    ksh93#2 0x5bb0a92305cd in initialize /home/johno/GitRepos/KornShell/ksh/src/lib/libast/port/astconf.c:591
    ksh93#3 0x5bb0a9230fac in format /home/johno/GitRepos/KornShell/ksh/src/lib/libast/port/astconf.c:632
    ksh93#4 0x5bb0a923ba4b in astgetconf /home/johno/GitRepos/KornShell/ksh/src/lib/libast/port/astconf.c:1382
    ksh93#5 0x5bb0a923d5c5 in astconf /home/johno/GitRepos/KornShell/ksh/src/lib/libast/port/astconf.c:1472
    ksh93#6 0x5bb0a924e07b in initconformance /home/johno/GitRepos/KornShell/ksh/src/lib/libast/misc/conformance.c:50
    ksh93#7 0x5bb0a924eff2 in conformance /home/johno/GitRepos/KornShell/ksh/src/lib/libast/misc/conformance.c:122
    ksh93#8 0x5bb0a9288b8e in b_cp /home/johno/GitRepos/KornShell/ksh/src/lib/libcmd/cp.c:706
    <CUT>

src/lib/libast/port/astconf.c:
- If fp->value and value pointed to the same allocated memory
  before realloc, avoid memcpy as value now points to freed memory.
- Produce an obvious panic if memory allocation fails.

src/cmd/ksh93/sh/init.c:
- For correctness, prevent memory leaks by freeing memory in sh_realloc
  upon failure.

src/cmd/ksh93/sh/init.c

@@ -245,7 +245,11 @@ void *sh_realloc(void *ptr, size_t size)
 	void *cp;
 	cp = realloc(ptr, size);
 	if(!cp)
+	{
+		if(ptr)
+			free(ptr);
 		nomemory(size);
+	}
 	return cp;
 }
 

src/lib/libast/port/astconf.c

@@ -306,6 +306,7 @@ synthesize(Feature_t* fp, const char* path, const char* value)
 	char*		d;
 	char*		v;
 	char*		p;
+	int		docpy;
 	int		n;
 
 #if DEBUG_astconf
@@ -458,14 +459,26 @@ synthesize(Feature_t* fp, const char* path, const char* value)
 		fp->value = 0;
 	if (n == 1 && (*value == '0' || *value == '-'))
 		n = 0;
-	if (!(fp->value = newof(fp->value, char, n, 1)))
-		fp->value = null;
+	docpy = fp->value != value;
+	if (!fp->value && !(fp->value = calloc(1, n + 1)))
+	{
+		error(ERROR_SYSTEM|ERROR_PANIC,"out of memory");
+		UNREACHABLE();
+	}
 	else
 	{
-		fp->flags |= CONF_ALLOC;
-		memcpy(fp->value, value, n);
-		fp->value[n] = 0;
+		char *tofree = fp->value;
+		if (!(fp->value = realloc(fp->value, n + 1)))
+		{
+			free(tofree);
+			error(ERROR_SYSTEM|ERROR_PANIC,"out of memory");
+			UNREACHABLE();
+		}
 	}
+	fp->flags |= CONF_ALLOC;
+	if(docpy)
+		memcpy(fp->value, value, n);
+	fp->value[n] = 0;
 	return fp->value;
 }