fix issue #337 - crash on start with armhf arch

Fix "objects[0] class #1 not in order!" panic.  The new check to
make sure that the elements of objects[] were in ascending order
by object class uses a plain 'char' index so -1 to indicate 'no
previous value' didn't work on a system using unsigned chars.

Verfied by temporarily adding '-funsigned-char' to CFLAGS before
and after the revision.  Before: panic, after: no panic.

Fixes #337

doc/fixes37.0

@@ -210,6 +210,7 @@ poison gas clouds located over known but unlit pools were visible as known
 after the wish parsing change, wishing for "<something of monster>" or for
 	"<monster something>" worked as intended but wishing for "<monster>"
 	(where <monster> used the canonical spelling) triggered a crash
+fix new "objects[0] class #1 not in order!" panic if plain 'char' is unsigned
 tty: redraw unexplored locations as S_unexplored rather than <space> after
 	map has been partially overwritten by popup menu or text display
 tty: previous change resulted in remnants of previous level being shown on

src/o_init.c

@@ -108,8 +108,8 @@ boolean domaterial;
 void
 init_objects()
 {
-    int i, first, last, sum;
-    char oclass, prevoclass;
+    int i, first, last, sum, prevoclass;
+    char oclass;
 #ifdef TEXTCOLOR
 #define COPY_OBJ_DESCR(o_dst, o_src) \
     o_dst.oc_descr_idx = o_src.oc_descr_idx, o_dst.oc_color = o_src.oc_color
@@ -138,7 +138,7 @@ init_objects()
          * earlier class would involve switching back to a lower class
          * number after having moved on to one or more other classes.
          */
-        if (oclass < prevoclass)
+        if ((int) oclass < prevoclass)
             panic("objects[%d] class #%d not in order!", first, oclass);
 
         last = first + 1;
@@ -181,7 +181,7 @@ init_objects()
         if (sum != 1000)
             error("init-prob error for class %d (%d%%)", oclass, sum);
         first = last;
-        prevoclass = oclass;
+        prevoclass = (int) oclass;
     }
     /* extra entry allows deriving the range of a class via
        bases[class] through bases[class+1]-1 for all classes */