pgcrypto: Remove static storage class from variables

Variables p, sp and ep were labeled with static storage class
but are all assigned before use so they cannot carry any data
across calls.  Fix by removing the static label.

Also while in there, make the magic variable const as it will
never change.

Author: Japin Li <japinli@hotmail.com>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/ME0P300MB0445096B67ACE8CE25772F00B6F72@ME0P300MB0445.AUSP300.PROD.OUTLOOK.COM

contrib/pgcrypto/crypt-md5.c

@@ -33,11 +33,11 @@ _crypt_to64(char *s, unsigned long v, int n)
 char *
 px_crypt_md5(const char *pw, const char *salt, char *passwd, unsigned dstlen)
 {
-	static char *magic = "$1$"; /* This string is magic for this algorithm.
+	static const char *magic = "$1$";	/* This string is magic for this
-								 * Having it this way, we can get better later
+										 * algorithm. Having it this way, we
-								 * on */
+										 * can get better later on */
-	static char *p;
+	char	   *p;
-	static const char *sp,
+	const char *sp,
 			   *ep;
 	unsigned char final[MD5_SIZE];
 	int			sl,
@@ -81,7 +81,7 @@ px_crypt_md5(const char *pw, const char *salt, char *passwd, unsigned dstlen)
 	px_md_update(ctx, (const uint8 *) pw, strlen(pw));
 
 	/* Then our magic string */
-	px_md_update(ctx, (uint8 *) magic, strlen(magic));
+	px_md_update(ctx, (const uint8 *) magic, strlen(magic));
 
 	/* Then the raw salt */
 	px_md_update(ctx, (const uint8 *) sp, sl);