Use "g" not "f" format in ecpg's PGTYPESnumeric_from_double().

The previous coding could overrun the provided buffer size for a very large
input, or lose precision for a very small input.  Adopt the methodology
that's been in use in the equivalent backend code for a long time.

Per private report from Bas van Schaik.  Back-patch to all supported
branches.

src/interfaces/ecpg/pgtypeslib/numeric.c

@@ -2,6 +2,7 @@
 
 #include "postgres_fe.h"
 #include <ctype.h>
+#include <float.h>
 #include <limits.h>
 
 #include "extern.h"
@@ -1497,11 +1498,11 @@ PGTYPESnumeric_copy(numeric *src, numeric *dst)
 int
 PGTYPESnumeric_from_double(double d, numeric *dst)
 {
-	char		buffer[100];
+	char		buffer[DBL_DIG + 100];
 	numeric    *tmp;
 	int			i;
 
-	if (sprintf(buffer, "%f", d) == 0)
+	if (sprintf(buffer, "%.*g", DBL_DIG, d) <= 0)
 		return -1;
 
 	if ((tmp = PGTYPESnumeric_from_asc(buffer, NULL)) == NULL)