Fix unportable usage of <ctype.h> functions.

isdigit(), isspace(), etc are likely to give surprising results if passed a
signed char.  We should always cast the argument to unsigned char to avoid
that.  Error in commit 63d6b97f, found by buildfarm member gaur.
Back-patch to 9.3, like that commit.

src/interfaces/ecpg/ecpglib/data.c

@@ -57,7 +57,7 @@ garbage_left(enum ARRAY_TYPE isarray, char **scan_length, enum COMPAT_MODE compa
 			/* skip invalid characters */
 			do {
 				(*scan_length)++;
-			} while (isdigit(**scan_length));
+			} while (isdigit((unsigned char) **scan_length));
 		}
 
 		if (**scan_length != ' ' && **scan_length != '\0')