simplify vasprintf implementation

the old implementation preallocated a buffer in order to try to avoid
calling vsnprintf more than once. not only did this potentially lead
to memory fragmentation from trimming with realloc; it also pulled in
realloc/free, which otherwise might not be needed in a static linked
program.

src/stdio/vasprintf.c

@@ -3,26 +3,13 @@
 #include <stdarg.h>
 #include <stdlib.h>
 
-#define GUESS 240U
-
 int vasprintf(char **s, const char *fmt, va_list ap)
 {
 	va_list ap2;
-	char *a;
-	int l=GUESS;
-
-	if (!(a=malloc(GUESS))) return -1;
-
 	va_copy(ap2, ap);
-	l=vsnprintf(a, GUESS, fmt, ap2);
+	int l = vsnprintf(0, 0, fmt, ap2);
 	va_end(ap2);
 
-	if (l<GUESS) {
-		char *b = realloc(a, l+1U);
-		*s = b ? b : a;
-		return l;
-	}
-	free(a);
 	if (l<0 || !(*s=malloc(l+1U))) return -1;
 	return vsnprintf(*s, l+1U, fmt, ap);
 }