revert removal of hex_decode() from ecpg from commit c3826f831e

ecpglib on certain platforms can't handle the pg_log_fatal calls from
libraries.  This was reported by the buildfarm.  It needs a refactoring
and return value change if it is later removed.

Backpatch-through: master

src/interfaces/ecpg/ecpglib/data.c

@@ -5,7 +5,6 @@
 
 #include <math.h>
 
-#include "common/hex_decode.h"
 #include "ecpgerrno.h"
 #include "ecpglib.h"
 #include "ecpglib_extern.h"
@@ -137,6 +136,57 @@ ecpg_hex_dec_len(unsigned srclen)
 	return srclen >> 1;
 }
 
+static inline char
+get_hex(char c)
+{
+	static const int8 hexlookup[128] = {
+		-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
+		-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
+		-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
+		0, 1, 2, 3, 4, 5, 6, 7, 8, 9, -1, -1, -1, -1, -1, -1,
+		-1, 10, 11, 12, 13, 14, 15, -1, -1, -1, -1, -1, -1, -1, -1, -1,
+		-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
+		-1, 10, 11, 12, 13, 14, 15, -1, -1, -1, -1, -1, -1, -1, -1, -1,
+		-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
+	};
+	int			res = -1;
+
+	if (c > 0 && c < 127)
+		res = hexlookup[(unsigned char) c];
+
+	return (char) res;
+}
+
+static unsigned
+hex_decode(const char *src, unsigned len, char *dst)
+{
+	const char *s,
+			   *srcend;
+	char		v1,
+				v2,
+			   *p;
+
+	srcend = src + len;
+	s = src;
+	p = dst;
+	while (s < srcend)
+	{
+		if (*s == ' ' || *s == '\n' || *s == '\t' || *s == '\r')
+		{
+			s++;
+			continue;
+		}
+		v1 = get_hex(*s++) << 4;
+		if (s >= srcend)
+			return -1;
+
+		v2 = get_hex(*s++);
+		*p++ = v1 | v2;
+	}
+
+	return p - dst;
+}
+
 unsigned
 ecpg_hex_encode(const char *src, unsigned len, char *dst)
 {