bugfix: nested if-condition

Otherwise, debug-execution yielded
```
Caught signal 8 (Floating point exception: floating-point invalid
operation)
```

Author: jjokella

src/clm5/biogeochem/ch4Mod.F90

@@ -2473,12 +2473,14 @@ subroutine ch4_prod (bounds, num_methc, filter_methc, num_methp, &
             end if
 
             ! If switched on, use pH factor for production based on spatial pH data defined in surface data.
-            if (.not. lake .and. usephfact .and. pH(c) >  pHmin .and.pH(c) <  pHmax) then
+            if (.not. lake .and. usephfact) then
+             if (pH(c) >  pHmin .and.pH(c) <  pHmax) then
                pH_fact_ch4 = 10._r8**(-0.2235_r8*pH(c)*pH(c) + 2.7727_r8*pH(c) - 8.6_r8)
                ! fitted function using data from Dunfield et al. 1993  
                ! Strictly less than one, with optimum at 6.5
                ! From Lei Meng
                f_ch4_adj = f_ch4_adj * pH_fact_ch4
+             end if
             else
                ! if no data, then no pH effects
             end if