vfscanf: fix possible invalid free due to uninitialized variable use

vfscanf() may use the variable 'alloc' uninitialized when taking the
branch introduced by commit b287cd745c2243f8e5114331763a5a9813b5f6ee.
Spotted by clang.

src/stdio/vfscanf.c

@@ -57,7 +57,7 @@ int vfscanf(FILE *restrict f, const char *restrict fmt, va_list ap)
 {
 	int width;
 	int size;
-	int alloc;
+	int alloc = 0;
 	int base;
 	const unsigned char *p;
 	int c, t;