Avoid a compiler warning.

Avoid copying "jeprof" to a 1-byte buffer within prof_boot0() when heap
profiling is disabled.  Although this is dead code under such
conditions, the compiler doesn't figure that part out.

Reported by Eduardo Silva.

include/jemalloc/internal/prof.h

@@ -8,7 +8,11 @@ typedef struct prof_ctx_s prof_ctx_t;
 typedef struct prof_tdata_s prof_tdata_t;
 
 /* Option defaults. */
+#ifdef JEMALLOC_PROF
 #  define PROF_PREFIX_DEFAULT		"jeprof"
+#else
+#  define PROF_PREFIX_DEFAULT		""
+#endif
 #define	LG_PROF_SAMPLE_DEFAULT		19
 #define	LG_PROF_INTERVAL_DEFAULT	-1