Fix some bogus direct uses of realloc().

pg_dump/parallel.c was using realloc() directly with no error check.
While the odds of an actual failure here seem pretty low, Coverity
complains about it, so fix by using pg_realloc() instead.

While looking for other instances, I noticed a couple of places in
psql that hadn't gotten the memo about the availability of pg_realloc.
These aren't bugs, since they did have error checks, but verbosely
inconsistent code is not a good thing.

Back-patch as far as 9.3.  9.2 did not have pg_dump/parallel.c, nor
did it have pg_realloc available in all frontend code.

src/bin/pg_dump/parallel.c

@@ -1300,7 +1300,7 @@ readMessageFromPipe(int fd)
 		{
 			/* could be any number */
 			bufsize += 16;
-			msg = (char *) realloc(msg, bufsize);
+			msg = (char *) pg_realloc(msg, bufsize);
 		}
 	}
 
@@ -1308,7 +1308,7 @@ readMessageFromPipe(int fd)
 	 * Worker has closed the connection, make sure to clean up before return
 	 * since we are not returning msg (but did allocate it).
 	 */
-	free(msg);
+	pg_free(msg);
 
 	return NULL;
 }

src/bin/psql/command.c

@@ -1112,12 +1112,7 @@ exec_command(const char *cmd,
 			while ((opt = psql_scan_slash_option(scan_state,
 												 OT_NORMAL, NULL, false)))
 			{
-				newval = realloc(newval, strlen(newval) + strlen(opt) + 1);
-				if (!newval)
-				{
-					psql_error("out of memory\n");
-					exit(EXIT_FAILURE);
-				}
+				newval = pg_realloc(newval, strlen(newval) + strlen(opt) + 1);
 				strcat(newval, opt);
 				free(opt);
 			}

src/bin/psql/tab-complete.c

@@ -3820,13 +3820,8 @@ complete_from_variables(char *text, const char *prefix, const char *suffix)
 		if (nvars >= maxvars)
 		{
 			maxvars *= 2;
-			varnames = (char **) realloc(varnames,
-										 (maxvars + 1) * sizeof(char *));
-			if (!varnames)
-			{
-				psql_error("out of memory\n");
-				exit(EXIT_FAILURE);
-			}
+			varnames = (char **) pg_realloc(varnames,
+											(maxvars + 1) * sizeof(char *));
 		}
 
 		buffer = (char *) pg_malloc(strlen(ptr->name) + overhead);