warnings: fix warnings on newer compilers

Mostly turning the crank on fixing simple warnings: arrays, for
instance, can never be nil.  A couple of pointers should have been
initialized to `nil` before being tested.

Some logic in `troff` was simplified: basically, an `if` statement had
a condition that must have always been true if that section of code
were being executed at all.

src/cmd/proof/font.c

@@ -177,8 +177,8 @@ loadfontname(int n, char *s)
 
 	if (strcmp(s, fname[n]) == 0)
 		return;
-	if(fname[n] && fname[n][0]){
-		if(lastload[n] && strcmp(lastload[n], fname[n]) == 0)
+	if(fname[n][0]){
+		if(strcmp(lastload[n], fname[n]) == 0)
 			return;
 		strcpy(lastload[n], fname[n]);
 	}

src/cmd/spell/sprog.c

@@ -985,7 +985,7 @@ trypref(char* ep, char* a, int lev, int flag)
 		deriv[lev+1].type += PREF;
 		h = tryword(bp,ep,lev+1,flag);
 		if(Set(h,NOPREF) ||
-		   ((tp->flag&IN) && inun(bp-2,h)==0)) {
+		   ((tp->flag&IN) && bp>=(word+2) && inun(bp-2,h)==0)) {
 			h = 0;
 			break;
 		}

src/cmd/tbl/t8.c

@@ -71,7 +71,7 @@ putline(int i, int nl)
 		ss = table[nl][c].col;
 		if (ss == 0)
 			continue;
-		if(font[c][stynum[nl]])
+		if(c < MAXHEAD && 0 <= nl && nl < 2)
 			chfont = 1;
 		if (point(ss) )
 			continue;

src/cmd/troff/n7.c

@@ -1,5 +1,6 @@
 #define _BSD_SOURCE 1	/* isascii */
 #define _DEFAULT_SOURCE 1
+#include <u.h>
 #include "tdef.h"
 #include "fns.h"
 #include "ext.h"
@@ -323,13 +324,14 @@ void ckul(void)
 void storeline(Tchar c, int w)
 {
 	int diff;
+	Tchar *nline;
 
 	if (linep >= line + lnsize - 2) {
 		lnsize += LNSIZE;
 		diff = linep - line;
-		if (( line = (Tchar *)realloc((char *)line, lnsize * sizeof(Tchar))) != NULL) {
-			if (linep && diff)
-				linep = line + diff;
+		if ((nline = (Tchar *)realloc((char *)line, lnsize * sizeof(Tchar))) != NULL) {
+			line = nline;
+			linep = line + diff;
 		} else {
 			if (over) {
 				return;
@@ -767,24 +769,26 @@ rtn:
 
 void storeword(Tchar c, int w)
 {
-	Tchar *savp;
+	Tchar *nword;
+	uintptr savp;
 	int i;
 
 	if (wordp >= word + wdsize - 2) {
 		wdsize += WDSIZE;
-		savp = word;
-		if (( word = (Tchar *)realloc((char *)word, wdsize * sizeof(Tchar))) != NULL) {
+		savp = (uintptr)word;
+		if ((nword = (Tchar *)realloc((char *)word, wdsize * sizeof(Tchar))) != NULL) {
+			word = nword;
 			if (wordp)
-				wordp = word + (wordp - savp);
+				wordp = word + ((uintptr)wordp - savp);
 			if (pendw)
-				pendw = word + (pendw - savp);
+				pendw = word + ((uintptr)pendw - savp);
 			if (wdstart)
-				wdstart = word + (wdstart - savp);
+				wdstart = word + ((uintptr)wdstart - savp);
 			if (wdend)
-				wdend = word + (wdend - savp);
+				wdend = word + ((uintptr)wdend - savp);
 			for (i = 0; i < NHYP; i++)
 				if (hyptr[i])
-					hyptr[i] = word + (hyptr[i] - savp);
+					hyptr[i] = word + ((uintptr)hyptr[i] - savp);
 		} else {
 			if (over) {
 				return;

src/libplumb/mesg.c

@@ -304,9 +304,11 @@ plumbunpackpartial(char *buf, int n, int *morep)
 	i = plumbline(&m->dst, buf, i, n, &bad);
 	i = plumbline(&m->wdir, buf, i, n, &bad);
 	i = plumbline(&m->type, buf, i, n, &bad);
+	attr = nil;
 	i = plumbline(&attr, buf, i, n, &bad);
 	m->attr = plumbunpackattr(attr);
 	free(attr);
+	ntext = nil;
 	i = plumbline(&ntext, buf, i, n, &bad);
 	m->ndata = atoi(ntext);
 	if(m->ndata != n-i){